mus: Disable external begin-frame source in the renderer.

content/renderer/gpu/render_widget_compositor.cc

Index: content/renderer/gpu/render_widget_compositor.cc
diff --git a/content/renderer/gpu/render_widget_compositor.cc b/content/renderer/gpu/render_widget_compositor.cc
index ce6ea32b5c014b0713a6c4e495e023a85c361186..625418242e7e398bd75059b39c1e610e97fcf730 100644
--- a/content/renderer/gpu/render_widget_compositor.cc
+++ b/content/renderer/gpu/render_widget_compositor.cc
@@ -512,7 +512,8 @@ cc::LayerTreeSettings RenderWidgetCompositor::GenerateLayerTreeSettings(
   settings.use_cached_picture_raster =
       !cmd.HasSwitch(cc::switches::kDisableCachedPictureRaster);
 
-  if (cmd.HasSwitch(switches::kUseRemoteCompositing))
+  if (cmd.HasSwitch(switches::kUseRemoteCompositing) ||
+      cmd.HasSwitch(switches::kIsRunningInMash))
     settings.use_external_begin_frame_source = false;
 
   return settings;