cc: Making BeginFrameSources support multiple BeginFrameObservers.

cc/scheduler/begin_frame_source.h

Index: cc/scheduler/begin_frame_source.h
diff --git a/cc/scheduler/begin_frame_source.h b/cc/scheduler/begin_frame_source.h
index d845dacd8e417505e27b3663246ae38c53234285..1131d72d1b3330f6a78a85fa21aca56c828db237 100644
--- a/cc/scheduler/begin_frame_source.h
+++ b/cc/scheduler/begin_frame_source.h
@@ -47,10 +47,10 @@ class CC_EXPORT BeginFrameObserver {
   // These requirements are designed to allow chaining and nesting of
   // BeginFrameObservers which filter the incoming BeginFrame messages while
   // preventing "double dropping" and other bad side effects.
-  virtual const BeginFrameArgs LastUsedBeginFrameArgs() const = 0;
+  virtual const BeginFrameArgs LastUsedBeginFrameArgs() = 0;
 
   // Tracing support
-  virtual void AsValueInto(base::trace_event::TracedValue* dict) const = 0;
+  virtual void AsValueInto(base::trace_event::TracedValue* dict) = 0;
 };
 
 // Simple mix in which implements a BeginFrameObserver which checks the
@@ -72,10 +72,10 @@ class CC_EXPORT BeginFrameObserverMixIn : public BeginFrameObserver {
   // OnBeginFrameMixInDelegate and updates the last_begin_frame_args_ value on
   // true.
   void OnBeginFrame(const BeginFrameArgs& args) override;
-  const BeginFrameArgs LastUsedBeginFrameArgs() const override;
+  const BeginFrameArgs LastUsedBeginFrameArgs() override;
 
   // Outputs last_begin_frame_args_
-  void AsValueInto(base::trace_event::TracedValue* dict) const override;
+  void AsValueInto(base::trace_event::TracedValue* dict) override;
 
  protected:
   // Subclasses should override this method!
@@ -122,7 +122,7 @@ class CC_EXPORT BeginFrameSource {
 
   // Tracing support - Recommend (but not required) to call this implementation
   // in any override.
-  virtual void AsValueInto(base::trace_event::TracedValue* dict) const = 0;
+  virtual void AsValueInto(base::trace_event::TracedValue* dict) = 0;
 };
 
 // Simple mix in which implements a BeginFrameSource.
@@ -134,7 +134,7 @@ class CC_EXPORT BeginFrameSource {
 //    in their own AsValueInto implementation.
 class CC_EXPORT BeginFrameSourceMixIn : public BeginFrameSource {
  public:
-  ~BeginFrameSourceMixIn() override {}
+  ~BeginFrameSourceMixIn() override;
 
   // BeginFrameSource
   bool NeedsBeginFrames() const final;
@@ -146,7 +146,7 @@ class CC_EXPORT BeginFrameSourceMixIn : public BeginFrameSource {
 
   // Tracing support - Recommend (but not required) to call this implementation
   // in any override.
-  void AsValueInto(base::trace_event::TracedValue* dict) const override;
+  void AsValueInto(base::trace_event::TracedValue* dict) override;
 
  protected:
   BeginFrameSourceMixIn();
@@ -159,7 +159,7 @@ class CC_EXPORT BeginFrameSourceMixIn : public BeginFrameSource {
   // needs_begin_frames change.
   virtual void OnNeedsBeginFramesChange(bool needs_begin_frames) {}
 
-  BeginFrameObserver* observer_;
+  ObserverList<BeginFrameObserver> observer_list_;
   bool needs_begin_frames_;
 
  private:
@@ -178,7 +178,7 @@ class CC_EXPORT BackToBackBeginFrameSource : public BeginFrameSourceMixIn {
   void DidFinishFrame(size_t remaining_frames) override;
 
   // Tracing
-  void AsValueInto(base::trace_event::TracedValue* dict) const override;
+  void AsValueInto(base::trace_event::TracedValue* dict) override;
 
  protected:
   explicit BackToBackBeginFrameSource(
@@ -211,7 +211,7 @@ class CC_EXPORT SyntheticBeginFrameSource : public BeginFrameSourceMixIn,
   ~SyntheticBeginFrameSource() override;
 
   // Tracing
-  void AsValueInto(base::trace_event::TracedValue* dict) const override;
+  void AsValueInto(base::trace_event::TracedValue* dict) override;
 
   // VSyncParameterObserver
   void OnUpdateVSyncParameters(base::TimeTicks new_vsync_timebase,
@@ -254,7 +254,7 @@ class CC_EXPORT BeginFrameSourceMultiplexer : public BeginFrameSourceMixIn,
   // calls to preserve the monotonicity of the BeginFrameArgs when switching
   // sources.
   void OnBeginFrame(const BeginFrameArgs& args) override;
-  const BeginFrameArgs LastUsedBeginFrameArgs() const override;
+  const BeginFrameArgs LastUsedBeginFrameArgs() override;
 
   // BeginFrameSource
   void DidFinishFrame(size_t remaining_frames) override;
@@ -263,7 +263,7 @@ class CC_EXPORT BeginFrameSourceMultiplexer : public BeginFrameSourceMixIn,
   void OnNeedsBeginFramesChange(bool needs_begin_frames) override;
 
   // Tracing
-  void AsValueInto(base::trace_event::TracedValue* dict) const override;
+  void AsValueInto(base::trace_event::TracedValue* dict) override;
 
  protected:
   BeginFrameSourceMultiplexer();