Aura-Mus: Enable Surface Synchronization behind flag

ui/aura/mus/window_tree_client.cc

Index: ui/aura/mus/window_tree_client.cc
diff --git a/ui/aura/mus/window_tree_client.cc b/ui/aura/mus/window_tree_client.cc
index c3bdece523a654b38ceff5cfb6df6c1b0b94852b..962ca17548696de081739af417d3c9249c5e8d1a 100644
--- a/ui/aura/mus/window_tree_client.cc
+++ b/ui/aura/mus/window_tree_client.cc
@@ -1300,7 +1300,16 @@ void WindowTreeClient::OnWindowSurfaceChanged(
   WindowMus* window = GetWindowByServerId(window_id);
   if (!window)
     return;
-  window->SetPrimarySurfaceInfo(surface_info);
+  if (enable_surface_synchronization_) {
+    // If surface synchronization is enabled, and the parent is informed
+    // of a child's surface then that surface ID is guaranteed to be available
+    // in the display compositor so we set it as the fallback.
+    window->SetFallbackSurfaceInfo(surface_info);

[sadrul, 2017/03/31 05:23:00]

Can you comment that the PrimarySurfaceInfo is set in WindowPortMus in this
case?

Shouldn't |surface_info| be set as the primary here too, though? Can you comment
on why not?

[Fady Samuel, 2017/03/31 12:51:27]

The parent allocates the LocalSurfaceId for the child and it manufactures the
primary SurfaceInfo. I've updated the comment to mention this. Thanks!

+  } else {
+    // If surface synchronization is disabled, fallback SurfaceInfos are never
+    // used.
+    window->SetPrimarySurfaceInfo(surface_info);
+  }
 }
 
 void WindowTreeClient::OnDragDropStart(