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

ui/ozone/platform/dri/gbm_surfaceless.cc

Index: ui/ozone/platform/dri/gbm_surfaceless.cc
diff --git a/ui/ozone/platform/dri/gbm_surfaceless.cc b/ui/ozone/platform/dri/gbm_surfaceless.cc
index b3748445a369b53d97866b759da0455c26cc4c0d..6942170ab13fcd07d345d2baaa51f7cf1e158452 100644
--- a/ui/ozone/platform/dri/gbm_surfaceless.cc
+++ b/ui/ozone/platform/dri/gbm_surfaceless.cc
@@ -38,6 +38,13 @@ bool GbmSurfaceless::OnSwapBuffers() {
   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_));
 }