Use input events to improve vsync scheduling

cc/vsync_time_source.h

Index: cc/vsync_time_source.h
diff --git a/cc/vsync_time_source.h b/cc/vsync_time_source.h
index ad412c0f289631e7d46b984a26622b3d4b623e37..7b9e57d8567d0b7cc8356dc0c7adbf8cff893f6e 100644
--- a/cc/vsync_time_source.h
+++ b/cc/vsync_time_source.h
@@ -13,6 +13,7 @@ namespace cc {
 class CC_EXPORT VSyncClient {
  public:
   virtual void DidVSync(base::TimeTicks frame_time) = 0;
+  virtual void DidReceiveLastInputEventForVSync() = 0;
 
  protected:
   virtual ~VSyncClient() {}
@@ -46,6 +47,10 @@ class CC_EXPORT VSyncTimeSource : public TimeSource, public VSyncClient {
 
   // VSyncClient implementation
   virtual void DidVSync(base::TimeTicks frame_time) OVERRIDE;
+  virtual void DidReceiveLastInputEventForVSync() OVERRIDE;
+
+  // Virtual for testing.
+  virtual base::TimeTicks Now() const;
 
  protected:
   explicit VSyncTimeSource(VSyncProvider* vsync_provider);
@@ -55,6 +60,7 @@ class CC_EXPORT VSyncTimeSource : public TimeSource, public VSyncClient {
   base::TimeDelta interval_;
   bool active_;
   bool notification_requested_;
+  bool did_synthesize_last_vsync_;
 
   VSyncProvider* vsync_provider_;
   TimeSourceClient* client_;