Revert of Removing #defines previously used for building without BoringSSL/OpenSSL. (patchset #2 id:20001 of https://codereview.webrtc.org/2640513002/ )

Reason for revert:
Broke chromium build, due to a config being removed. Will add it back and remove the dependency in a chromium CL.

Original issue's description:
> Removing #defines previously used for building without BoringSSL/OpenSSL.
>
> These defines don't work any more, so they only cause confusion:
>
> FEATURE_ENABLE_SSL
> HAVE_OPENSSL_SSL_H
> SSL_USE_OPENSSL
>
> BUG=webrtc:7025
>
> Review-Url: https://codereview.webrtc.org/2640513002
> Cr-Commit-Position: refs/heads/master@{#16196}
> Committed: eaa826c2ee

TBR=kjellander@webrtc.org
# Skipping CQ checks because original CL landed less than 1 days ago.
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
BUG=webrtc:7025

Review-Url: https://codereview.webrtc.org/2648003003
Cr-Commit-Position: refs/heads/master@{#16197}

webrtc/base/helpers.cc

@@ -13,7 +13,18 @@
 #include <limits>
 #include <memory>
 
+#if defined(FEATURE_ENABLE_SSL)
+#include "webrtc/base/sslconfig.h"
+#if defined(SSL_USE_OPENSSL)
 #include <openssl/rand.h>
+#else
+#if defined(WEBRTC_WIN)
+#define WIN32_LEAN_AND_MEAN
+#include <windows.h>
+#include <ntsecapi.h>
+#endif  // WEBRTC_WIN
+#endif  // else
+#endif  // FEATURE_ENABLED_SSL
 
 #include "webrtc/base/base64.h"
 #include "webrtc/base/basictypes.h"
@@ -34,6 +45,7 @@ class RandomGenerator {
   virtual bool Generate(void* buf, size_t len) = 0;
 };
 
+#if defined(SSL_USE_OPENSSL)
 // The OpenSSL RNG.
 class SecureRandomGenerator : public RandomGenerator {
  public:
@@ -45,6 +57,79 @@ class SecureRandomGenerator : public RandomGenerator {
   }
 };
 
+#else
+#if defined(WEBRTC_WIN)
+class SecureRandomGenerator : public RandomGenerator {
+ public:
+  SecureRandomGenerator() : advapi32_(NULL), rtl_gen_random_(NULL) {}
+  ~SecureRandomGenerator() {
+    FreeLibrary(advapi32_);
+  }
+
+  virtual bool Init(const void* seed, size_t seed_len) {
+    // We don't do any additional seeding on Win32, we just use the CryptoAPI
+    // RNG (which is exposed as a hidden function off of ADVAPI32 so that we
+    // don't need to drag in all of CryptoAPI)
+    if (rtl_gen_random_) {
+      return true;
+    }
+
+    advapi32_ = LoadLibrary(L"advapi32.dll");
+    if (!advapi32_) {
+      return false;
+    }
+
+    rtl_gen_random_ = reinterpret_cast<RtlGenRandomProc>(
+        GetProcAddress(advapi32_, "SystemFunction036"));
+    if (!rtl_gen_random_) {
+      FreeLibrary(advapi32_);
+      return false;
+    }
+
+    return true;
+  }
+  virtual bool Generate(void* buf, size_t len) {
+    if (!rtl_gen_random_ && !Init(NULL, 0)) {
+      return false;
+    }
+    return (rtl_gen_random_(buf, static_cast<int>(len)) != FALSE);
+  }
+
+ private:
+  typedef BOOL (WINAPI *RtlGenRandomProc)(PVOID, ULONG);
+  HINSTANCE advapi32_;
+  RtlGenRandomProc rtl_gen_random_;
+};
+
+#elif !defined(FEATURE_ENABLE_SSL)
+
+// No SSL implementation -- use rand()
+class SecureRandomGenerator : public RandomGenerator {
+ public:
+  virtual bool Init(const void* seed, size_t len) {
+    if (len >= 4) {
+      srand(*reinterpret_cast<const int*>(seed));
+    } else {
+      srand(*reinterpret_cast<const char*>(seed));
+    }
+    return true;
+  }
+  virtual bool Generate(void* buf, size_t len) {
+    char* bytes = reinterpret_cast<char*>(buf);
+    for (size_t i = 0; i < len; ++i) {
+      bytes[i] = static_cast<char>(rand());
+    }
+    return true;
+  }
+};
+
+#else
+
+#error No SSL implementation has been selected!
+
+#endif  // WEBRTC_WIN
+#endif
+
 // A test random generator, for predictable output.
 class TestRandomGenerator : public RandomGenerator {
  public: