Add FrameGenerator Unit Tests

cc/test/test_compositor_frame_sink.h

Index: cc/test/test_compositor_frame_sink.h
diff --git a/cc/test/test_compositor_frame_sink.h b/cc/test/test_compositor_frame_sink.h
index 9d97bc9053d71471835afbc4dac0b4adb353a03e..995f1afa20c6cffb137b867f61ab7d095e3781d9 100644
--- a/cc/test/test_compositor_frame_sink.h
+++ b/cc/test/test_compositor_frame_sink.h
@@ -94,6 +94,8 @@ class TestCompositorFrameSink : public CompositorFrameSink,
                               const RenderPassList& render_passes) override;
   void DisplayDidDrawAndSwap() override;
 
+  int numder_of_frames_submitted() { return numder_of_frames_submitted_; }
+
  private:
   // ExternalBeginFrameSource implementation.
   void OnNeedsBeginFrames(bool needs_begin_frames) override;