cc: Introduce BeginFrame sequence numbers and acknowledgements.

cc/surfaces/display.h

Index: cc/surfaces/display.h
diff --git a/cc/surfaces/display.h b/cc/surfaces/display.h
index a91ae09938d8d3d30673299793d2a7ce449492a6..5fa461eabd24cbea2def4c902c5aa86cb8e32757 100644
--- a/cc/surfaces/display.h
+++ b/cc/surfaces/display.h
@@ -11,7 +11,6 @@
 #include "base/macros.h"
 #include "cc/output/output_surface_client.h"
 #include "cc/resources/returned_resource.h"
-#include "cc/scheduler/begin_frame_source.h"
 #include "cc/surfaces/display_scheduler.h"
 #include "cc/surfaces/frame_sink_id.h"
 #include "cc/surfaces/surface_aggregator.h"
@@ -34,6 +33,7 @@ namespace cc {
 
 class BeginFrameSource;
 class DirectRenderer;
+class DisplayBeginFrameSource;
 class DisplayClient;
 class OutputSurface;
 class RendererSettings;
@@ -95,6 +95,13 @@ class CC_SURFACES_EXPORT Display : public DisplaySchedulerClient,
 
   void ForceImmediateDrawAndSwapIfPossible();
 
+  // Swap the Display's current BeginFrameSource with the provided one. The
+  // provided unique_ptr will be updated to point to the previous source. Used
+  // to set an ExternalBeginFrameSource for manual control over BeginFrames
+  // through DevTools.
+  void SwapBeginFrameSource(
+      std::unique_ptr<BeginFrameSource>* begin_frame_source);
+
  private:
   void InitializeRenderer();
   void UpdateRootSurfaceResourcesLocked();
@@ -117,7 +124,7 @@ class CC_SURFACES_EXPORT Display : public DisplaySchedulerClient,
 
   // The begin_frame_source_ is often known by the output_surface_ and
   // the scheduler_.
-  std::unique_ptr<BeginFrameSource> begin_frame_source_;
+  std::unique_ptr<DisplayBeginFrameSource> begin_frame_source_;
   std::unique_ptr<OutputSurface> output_surface_;
   std::unique_ptr<DisplayScheduler> scheduler_;
   std::unique_ptr<ResourceProvider> resource_provider_;