Test that main thread frames cease to fire during composited animation

cc/ipc/compositor_frame_metadata_struct_traits.h

Index: cc/ipc/compositor_frame_metadata_struct_traits.h
diff --git a/cc/ipc/compositor_frame_metadata_struct_traits.h b/cc/ipc/compositor_frame_metadata_struct_traits.h
index 46bb130d42b799e7350f17787c714980d6e9cb36..a66125efc392dcc51df7d800ed26bc7d37a05ef4 100644
--- a/cc/ipc/compositor_frame_metadata_struct_traits.h
+++ b/cc/ipc/compositor_frame_metadata_struct_traits.h
@@ -125,6 +125,11 @@ struct StructTraits<cc::mojom::CompositorFrameMetadataDataView,
     return metadata.frame_token;
   }
 
+  static bool is_main_thread_frame(
+      const cc::CompositorFrameMetadata& metadata) {
+    return metadata.is_main_thread_frame;
+  }
+
   static bool Read(cc::mojom::CompositorFrameMetadataDataView data,
                    cc::CompositorFrameMetadata* out);
 };