mus: Turn off using BeginFrameSource in ui::Compositor.

ui/compositor/compositor.cc

Index: ui/compositor/compositor.cc
diff --git a/ui/compositor/compositor.cc b/ui/compositor/compositor.cc
index 862b5fb3aee1f5eddc655530093e329357535334..7bb1f0957bd1a6948ecf3de60c9c07e6d4f7f9e6 100644
--- a/ui/compositor/compositor.cc
+++ b/ui/compositor/compositor.cc
@@ -45,6 +45,11 @@ namespace {
 const double kDefaultRefreshRate = 60.0;
 const double kTestRefreshRate = 200.0;
 
+bool IsRunningInMojoShell(base::CommandLine* command_line) {
+  const char kMojoShellFlag[] = "mojo-platform-channel-handle";
+  return command_line->HasSwitch(kMojoShellFlag);
+}
+
 }  // namespace
 
 namespace ui {
@@ -175,7 +180,10 @@ Compositor::Compositor(ui::ContextFactory* context_factory,
   // thread.
   settings.image_decode_tasks_enabled = false;
 
-  settings.use_output_surface_begin_frame_source = true;
+  // TODO(crbug.com/603600): This should always be turned on once mus tells its
+  // clients about BeginFrame.
+  settings.use_output_surface_begin_frame_source =
+      !IsRunningInMojoShell(command_line);
 
 #if !defined(OS_ANDROID)
   // TODO(sohanjg): Revisit this memory usage in tile manager.