[cc] Acknowledge BeginFrames in SurfaceDependencyTracker.

cc/surfaces/surface_dependency_tracker.h

Index: cc/surfaces/surface_dependency_tracker.h
diff --git a/cc/surfaces/surface_dependency_tracker.h b/cc/surfaces/surface_dependency_tracker.h
index 193eb31a22f8e13797abeb71ef30cc37cbe537e8..e5d2b13615b8afaa39bb55f11514e0dd34c1e335 100644
--- a/cc/surfaces/surface_dependency_tracker.h
+++ b/cc/surfaces/surface_dependency_tracker.h
@@ -71,6 +71,14 @@ class CC_SURFACES_EXPORT SurfaceDependencyTracker : public BeginFrameObserver,
   // corresponding to |surface_id|.
   void NotifySurfaceIdAvailable(const SurfaceId& surface_id);
 
+  // Determines whether we should be observing the BeginFrameSource and
+  // starts/stops observing as necessary.
+  void UpdateNeedsBeginFrames();
+
+  // Acknowledge and confirm the last BeginFrame we received after we have
+  // activated all previously pending frames.
+  void AcknowledgeLastBeginFrame();
+
   SurfaceManager* const surface_manager_;
 
   // The last begin frame args generated by the begin frame source.
@@ -89,6 +97,9 @@ class CC_SURFACES_EXPORT SurfaceDependencyTracker : public BeginFrameObserver,
 
   PendingSurfaceSet pending_surfaces_;
 
+  // Whether we are currently observing the BeginFrameSource.
+  bool needs_begin_frames_ = false;
+
   DISALLOW_COPY_AND_ASSIGN(SurfaceDependencyTracker);
 };