cc: Make BackToBackBeginFrameSource a SyntheticBeginFrameSource.

content/renderer/gpu/compositor_external_begin_frame_source.h

Index: content/renderer/gpu/compositor_external_begin_frame_source.h
diff --git a/content/renderer/gpu/compositor_external_begin_frame_source.h b/content/renderer/gpu/compositor_external_begin_frame_source.h
index 116529a229fe9241bbc6922f592e67aa670a8653..db1e4bc6a50ccdcffa59c3e707231d4d38524eec 100644
--- a/content/renderer/gpu/compositor_external_begin_frame_source.h
+++ b/content/renderer/gpu/compositor_external_begin_frame_source.h
@@ -21,7 +21,7 @@ namespace content {
 // This class can be created only on the main thread, but then becomes pinned
 // to a fixed thread where cc::Scheduler is running.
 class CompositorExternalBeginFrameSource
-    : public cc::BeginFrameSourceBase,
+    : public cc::BeginFrameSource,
       public NON_EXPORTED_BASE(base::NonThreadSafe) {
  public:
   explicit CompositorExternalBeginFrameSource(
@@ -30,9 +30,11 @@ class CompositorExternalBeginFrameSource
       int routing_id);
   ~CompositorExternalBeginFrameSource() override;
 
-  // cc::BeginFrameSourceBase implementation.
+  // cc::BeginFrameSource implementation.
   void AddObserver(cc::BeginFrameObserver* obs) override;
-  void OnNeedsBeginFramesChanged(bool needs_begin_frames) override;
+  void RemoveObserver(cc::BeginFrameObserver* obs) override;
+  void DidFinishFrame(cc::BeginFrameObserver* obs,
+                      size_t remaining_frames) override {}
 
  private:
   class CompositorExternalBeginFrameSourceProxy
@@ -61,6 +63,7 @@ class CompositorExternalBeginFrameSource
   void SetClientReady();
   void OnMessageReceived(const IPC::Message& message);
 
+  void OnSetBeginFrameSourcePaused(bool paused);
   void OnBeginFrame(const cc::BeginFrameArgs& args);
   bool Send(IPC::Message* message);
 
@@ -71,6 +74,8 @@ class CompositorExternalBeginFrameSource
   int routing_id_;
   CompositorForwardingMessageFilter::Handler begin_frame_source_filter_handler_;
   cc::BeginFrameArgs missed_begin_frame_args_;
+  std::set<cc::BeginFrameObserver*> observers_;
+  bool paused_ = false;
 
   DISALLOW_COPY_AND_ASSIGN(CompositorExternalBeginFrameSource);
 };