Hook up ui::Compositor to Display's BeginFrameSource

components/mus/surfaces/top_level_display_client.cc

Index: components/mus/surfaces/top_level_display_client.cc
diff --git a/components/mus/surfaces/top_level_display_client.cc b/components/mus/surfaces/top_level_display_client.cc
index c4f1de1941e2bbd82d80f7ff580f63e1a83995fa..b37a4010e02f575662aaedc876a94bf43afe6ff8 100644
--- a/components/mus/surfaces/top_level_display_client.cc
+++ b/components/mus/surfaces/top_level_display_client.cc
@@ -99,6 +99,10 @@ void TopLevelDisplayClient::OutputSurfaceLost() {
 void TopLevelDisplayClient::SetMemoryPolicy(
     const cc::ManagedMemoryPolicy& policy) {}
 
+cc::BeginFrameSource* TopLevelDisplayClient::BeginFrameSourceForChildren() {
+  return synthetic_frame_source_.get();
+}
+
 void TopLevelDisplayClient::OnVSyncParametersUpdated(int64_t timebase,
                                                      int64_t interval) {
   auto timebase_time_ticks = base::TimeTicks::FromInternalValue(timebase);