Test that main thread frames cease to fire during composited animation

cc/output/compositor_frame_metadata.h

Index: cc/output/compositor_frame_metadata.h
diff --git a/cc/output/compositor_frame_metadata.h b/cc/output/compositor_frame_metadata.h
index 98bed560715476f4dc71382cdc383574ba08b86b..1636becb11684e4fae1fab0b61448df4a344d62e 100644
--- a/cc/output/compositor_frame_metadata.h
+++ b/cc/output/compositor_frame_metadata.h
@@ -97,6 +97,8 @@ class CC_EXPORT CompositorFrameMetadata {
   // the embedder wants to do something after a particular frame is processed.
   uint32_t frame_token = 0;
 
+  bool is_main_thread_frame = true;
+
  private:
   CompositorFrameMetadata(const CompositorFrameMetadata& other);
   CompositorFrameMetadata operator=(const CompositorFrameMetadata&) = delete;