Revert of Delete rtc::linked_ptr. Only use, in statstypes.h, replaced bu std::unique_ptr. (patchset #1 id:1 of https://codereview.webrtc.org/2567143003/ )

Reason for revert:
The change from rtc::linked_ptr to std::unique_ptr broke a downstream project.

Original issue's description:
> Delete rtc::linked_ptr. Only use, in statstypes.h, replaced with std::unique_ptr.
>
> BUG=webrtc:6424
>
> Committed: https://crrev.com/36f74e55792cae19db8b222c29aa38d6e0eb1225
> Cr-Commit-Position: refs/heads/master@{#15588}

TBR=solenberg@webrtc.org,pthatcher@webrtc.org,hta@webrtc.org
# Skipping CQ checks because original CL landed less than 1 days ago.
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
BUG=webrtc:6424

Review-Url: https://codereview.webrtc.org/2576673002
Cr-Commit-Position: refs/heads/master@{#15589}

webrtc/api/statstypes.h

@@ -22,6 +22,7 @@
 #include "webrtc/base/basictypes.h"
 #include "webrtc/base/common.h"
 #include "webrtc/base/constructormagic.h"
+#include "webrtc/base/linked_ptr.h"
 #include "webrtc/base/refcount.h"
 #include "webrtc/base/scoped_ref_ptr.h"
 #include "webrtc/base/stringencode.h"
@@ -320,7 +321,7 @@ class StatsReport {
 
   // TODO(tommi): Consider using a similar approach to how we store Ids using
   // scoped_refptr for values.
-  typedef std::unique_ptr<Value> ValuePtr;
+  typedef rtc::linked_ptr<Value> ValuePtr;
   typedef std::map<StatsValueName, ValuePtr> Values;
 
   // Ownership of |id| is passed to |this|.

webrtc/base/BUILD.gn

@@ -427,6 +427,7 @@ rtc_static_library("rtc_base") {
     "httpcommon.h",
     "ipaddress.cc",
     "ipaddress.h",
+    "linked_ptr.h",
     "messagedigest.cc",
     "messagedigest.h",
     "messagehandler.cc",

webrtc/base/cryptstring.h

@@ -17,6 +17,8 @@
 #include <string>
 #include <vector>
 
+#include "webrtc/base/linked_ptr.h"
+
 namespace rtc {
 
 class CryptStringImpl {

webrtc/base/linked_ptr.h

@@ -0,0 +1,125 @@
+/*
+ *  Copyright 2004 The WebRTC Project Authors. All rights reserved.
+ *
+ *  Use of this source code is governed by a BSD-style license
+ *  that can be found in the LICENSE file in the root of the source
+ *  tree. An additional intellectual property rights grant can be found
+ *  in the file PATENTS.  All contributing project authors may
+ *  be found in the AUTHORS file in the root of the source tree.
+ */
+
+/*
+ * linked_ptr - simple reference linked pointer
+ * (like reference counting, just using a linked list of the references
+ * instead of their count.)
+ *
+ * The implementation stores three pointers for every linked_ptr, but
+ * does not allocate anything on the free store.
+ */
+
+#ifndef WEBRTC_BASE_LINKED_PTR_H__
+#define WEBRTC_BASE_LINKED_PTR_H__
+
+namespace rtc {
+
+/* For ANSI-challenged compilers, you may want to #define
+ * NO_MEMBER_TEMPLATES, explicit or mutable */
+#define NO_MEMBER_TEMPLATES
+
+template <class X> class linked_ptr
+{
+public:
+
+#ifndef NO_MEMBER_TEMPLATES
+#   define TEMPLATE_FUNCTION template <class Y>
+    TEMPLATE_FUNCTION friend class linked_ptr<Y>;
+#else
+#   define TEMPLATE_FUNCTION
+    typedef X Y;
+#endif
+
+    typedef X element_type;
+
+    explicit linked_ptr(X* p = 0) throw()
+        : itsPtr(p) {itsPrev = itsNext = this;}
+    ~linked_ptr()
+        {release();}
+    linked_ptr(const linked_ptr& r) throw()
+        {acquire(r);}
+    linked_ptr& operator=(const linked_ptr& r)
+    {
+        if (this != &r) {
+            release();
+            acquire(r);
+        }
+        return *this;
+    }
+
+#ifndef NO_MEMBER_TEMPLATES
+    template <class Y> friend class linked_ptr<Y>;
+    template <class Y> linked_ptr(const linked_ptr<Y>& r) throw()
+        {acquire(r);}
+    template <class Y> linked_ptr& operator=(const linked_ptr<Y>& r)
+    {
+        if (this != &r) {
+            release();
+            acquire(r);
+        }
+        return *this;
+    }
+#endif // NO_MEMBER_TEMPLATES
+
+    X& operator*()  const throw()   {return *itsPtr;}
+    X* operator->() const throw()   {return itsPtr;}
+    X* get()        const throw()   {return itsPtr;}
+    bool unique()   const throw()   {return itsPrev ? itsPrev==this : true;}
+
+private:
+    X*                          itsPtr;
+    mutable const linked_ptr*   itsPrev;
+    mutable const linked_ptr*   itsNext;
+
+    void acquire(const linked_ptr& r) throw()
+    { // insert this to the list
+        itsPtr = r.itsPtr;
+        itsNext = r.itsNext;
+        itsNext->itsPrev = this;
+        itsPrev = &r;
+#ifndef mutable
+        r.itsNext = this;
+#else // for ANSI-challenged compilers
+        (const_cast<linked_ptr<X>*>(&r))->itsNext = this;
+#endif
+    }
+
+#ifndef NO_MEMBER_TEMPLATES
+    template <class Y> void acquire(const linked_ptr<Y>& r) throw()
+    { // insert this to the list
+        itsPtr = r.itsPtr;
+        itsNext = r.itsNext;
+        itsNext->itsPrev = this;
+        itsPrev = &r;
+#ifndef mutable
+        r.itsNext = this;
+#else // for ANSI-challenged compilers
+        (const_cast<linked_ptr<X>*>(&r))->itsNext = this;
+#endif
+    }
+#endif // NO_MEMBER_TEMPLATES
+
+    void release()
+    { // erase this from the list, delete if unique
+        if (unique()) delete itsPtr;
+        else {
+            itsPrev->itsNext = itsNext;
+            itsNext->itsPrev = itsPrev;
+            itsPrev = itsNext = 0;
+        }
+        itsPtr = 0;
+    }
+};
+
+} // namespace rtc
+
+#endif // WEBRTC_BASE_LINKED_PTR_H__
+