[ozone] Add stub async swap buffers implementation to ozone surfaceless.

ui/ozone/platform/dri/gbm_surfaceless.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/ozone/platform/dri/gbm_surfaceless.h"
 
 #include "ui/ozone/platform/dri/dri_vsync_provider.h"
 #include "ui/ozone/platform/dri/dri_window_delegate.h"
 #include "ui/ozone/platform/dri/gbm_buffer.h"
 #include "ui/ozone/platform/dri/hardware_display_controller.h"
@@ skipping 20 matching lines @@
   HardwareDisplayController* controller = window_delegate_->GetController();
   if (!controller)
     return true;
 
   bool success = controller->SchedulePageFlip();
   controller->WaitForPageFlipEvent();
 
   return success;
 }
 
+bool GbmSurfaceless::OnSwapBuffersAsync(
+    const SwapCompletionCallback& callback) {
+  bool success = OnSwapBuffers();
+  callback.Run();
+  return success;
+}
+
 scoped_ptr<gfx::VSyncProvider> GbmSurfaceless::CreateVSyncProvider() {
   return scoped_ptr<gfx::VSyncProvider>(new DriVSyncProvider(window_delegate_));
 }
 
 }  // namespace ui