Fix Omnibox.CharTypedToRepaintLatency regression with D3DVsync experiment

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 98ea3771b397907cd7da351af4a4fd660d945134..d835dcc37db10e23bd6ec3407e7272e7152ddccb 100644
--- a/cc/scheduler/begin_frame_source.h
+++ b/cc/scheduler/begin_frame_source.h
@@ -275,6 +275,10 @@ class CC_EXPORT ExternalBeginFrameSource : public BeginFrameSource {
   void OnBeginFrame(const BeginFrameArgs& args);
 
  protected:
+  // Called on AddObserver and gets missed BeginFrameArgs for the given
+  // observer.
+  virtual BeginFrameArgs GetMissedBeginFrameArgs(BeginFrameObserver* obs);
+
   BeginFrameArgs last_begin_frame_args_;
   std::unordered_set<BeginFrameObserver*> observers_;
   ExternalBeginFrameSourceClient* client_;