Aura-Mus: Enable Surface Synchronization behind flag

ui/aura/mus/window_tree_client.cc

 // Copyright 2014 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "ui/aura/mus/window_tree_client.h"
 
 #include <stddef.h>
 
 #include <string>
 #include <utility>
@@ skipping 1282 matching lines @@
 
   window->SetPredefinedCursorFromServer(cursor);
 }
 
 void WindowTreeClient::OnWindowSurfaceChanged(
     Id window_id,
     const cc::SurfaceInfo& surface_info) {
   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);
+  } else {
+    // If surface synchronization is disabled, fallback SurfaceInfos are never
+    // used.
+    window->SetPrimarySurfaceInfo(surface_info);
+  }
 }
 
 void WindowTreeClient::OnDragDropStart(
     const std::unordered_map<std::string, std::vector<uint8_t>>& mime_data) {
   drag_drop_controller_->OnDragDropStart(mojo::UnorderedMapToMap(mime_data));
 }
 
 void WindowTreeClient::OnDragEnter(Id window_id,
                                    uint32_t key_state,
                                    const gfx::Point& position,
@@ skipping 634 matching lines @@
   return ScheduleInFlightChange(base::MakeUnique<InFlightCaptureChange>(
       this, capture_synchronizer_.get(), window));
 }
 
 uint32_t WindowTreeClient::CreateChangeIdForFocus(WindowMus* window) {
   return ScheduleInFlightChange(base::MakeUnique<InFlightFocusChange>(
       this, focus_synchronizer_.get(), window));
 }
 
 }  // namespace aura