Add support for WeakPtr<T>

The implementation is borrowed from Chromium.

Also change use of raw pointers in VideoSendStreamImpl to use WeakPtr<>

BUG= webrtc:6289

Review-Url: https://codereview.webrtc.org/2367853002
Cr-Commit-Position: refs/heads/master@{#14468}

webrtc/base/weak_ptr.cc

@@ -0,0 +1,87 @@
+/*
+ *  Copyright 2016 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.
+ */
+
+#include "webrtc/base/weak_ptr.h"
+
+// The implementation is borrowed from chromium except that it does not
+// implement SupportsWeakPtr.
+
+namespace rtc {
+namespace internal {
+
+WeakReference::Flag::Flag() : is_valid_(true) {
+  // Flags only become bound when checked for validity, or invalidated,
+  // so that we can check that later validity/invalidation operations on
+  // the same Flag take place on the same sequence.
+  checker_.Detach();
+}
+
+void WeakReference::Flag::Invalidate() {
+  RTC_DCHECK(checker_.CalledSequentially())
+      << "WeakPtrs must be invalidated on the same sequence.";
+  is_valid_ = false;
+}
+
+bool WeakReference::Flag::IsValid() const {
+  RTC_DCHECK(checker_.CalledSequentially())
+      << "WeakPtrs must be checked on the same sequence.";
+  return is_valid_;
+}
+
+WeakReference::Flag::~Flag() {}
+
+WeakReference::WeakReference() {}
+
+WeakReference::WeakReference(const Flag* flag) : flag_(flag) {}
+
+WeakReference::~WeakReference() {}
+
+WeakReference::WeakReference(WeakReference&& other) = default;
+
+WeakReference::WeakReference(const WeakReference& other) = default;
+
+bool WeakReference::is_valid() const {
+  return flag_.get() && flag_->IsValid();
+}
+
+WeakReferenceOwner::WeakReferenceOwner() {
+  checker_.Detach();
+}
+
+WeakReferenceOwner::~WeakReferenceOwner() {
+  RTC_DCHECK(checker_.CalledSequentially());
+  Invalidate();
+}
+
+WeakReference WeakReferenceOwner::GetRef() const {
+  RTC_DCHECK(checker_.CalledSequentially());
+  // If we hold the last reference to the Flag then create a new one.
+  if (!HasRefs())
+    flag_ = new RefCountedObject<WeakReference::Flag>();
+
+  return WeakReference(flag_.get());
+}
+
+void WeakReferenceOwner::Invalidate() {
+  RTC_DCHECK(checker_.CalledSequentially());
+  if (flag_.get()) {
+    flag_->Invalidate();
+    flag_ = NULL;
+  }
+}
+
+WeakPtrBase::WeakPtrBase() {}
+
+WeakPtrBase::~WeakPtrBase() {}
+
+WeakPtrBase::WeakPtrBase(const WeakReference& ref) : ref_(ref) {}
+
+}  // namespace internal
+}  // namespace rtc