mus: Make sure window parenting happens in the window server.

services/ui/gpu/interfaces/gpu_host.mojom

 // Copyright 2016 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.
 
 module ui.mojom;
 
 import "gpu/ipc/common/gpu_info.mojom";
+import "gpu/ipc/common/surface_handle.mojom";
 import "services/ui/gpu/interfaces/context_lost_reason.mojom";
 import "url/mojo/url.mojom";
 
 interface GpuHost {
   DidInitialize(gpu.mojom.GpuInfo gpu_info);
   DidCreateOffscreenContext(url.mojom.Url url);
   DidDestroyOffscreenContext(url.mojom.Url url);
 
   DidDestroyChannel(int32 client_id);
   DidLoseContext(bool offscreen,
                  ContextLostReason reason,
                  url.mojom.Url active_url);
 
+  SetChildSurface(gpu.mojom.SurfaceHandle parent,
+                  gpu.mojom.SurfaceHandle child);
   StoreShaderToDisk(int32 client_id, string key, string shader);
 };