(Reland) Discard compositor frames from unloaded web content

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 381edb3f45cdacd533a48d382940adfc1203f69c..c58ba45b4b2cdbf6ee0d2f966cf8b3c7ca80a069 100644
--- a/cc/ipc/compositor_frame_metadata_struct_traits.h
+++ b/cc/ipc/compositor_frame_metadata_struct_traits.h
@@ -111,6 +111,11 @@ struct StructTraits<cc::mojom::CompositorFrameMetadataDataView,
     return metadata.can_activate_before_dependencies;
   }
 
+  static uint32_t content_source_id(
+      const cc::CompositorFrameMetadata& metadata) {
+    return metadata.content_source_id;
+  }
+
   static bool Read(cc::mojom::CompositorFrameMetadataDataView data,
                    cc::CompositorFrameMetadata* out);
 };