cc: Introduce BeginFrame sequence numbers and acknowledgements.

cc/scheduler/scheduler.h

Index: cc/scheduler/scheduler.h
diff --git a/cc/scheduler/scheduler.h b/cc/scheduler/scheduler.h
index afc2247ec02d44af1bcdc54b6f6d85d2168ff2a5..6741ca75fb05b227383af260a264e4f97cd215fa 100644
--- a/cc/scheduler/scheduler.h
+++ b/cc/scheduler/scheduler.h
@@ -142,6 +142,10 @@ class CC_EXPORT Scheduler : public BeginFrameObserverBase {
     return begin_frame_source_;
   }
 
+  uint64_t CurrentBeginFrameSourceId();
+  uint64_t CurrentBeginFrameNumber();
+  uint64_t OldestIncorporatedFrameForActiveTree();
+
  protected:
   // Virtual for testing.
   virtual base::TimeTicks Now() const;