services/ui: Introduce mojom.GpuServiceInternal.

services/ui/gpu/interfaces/gpu_service_internal.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";
+
+interface GpuServiceInternal {
+  Initialize() => (gpu.mojom.GpuInfo gpu_info);
+
+  // Tells the GPU service to create a new channel for communication with a
+  // client. The GPU service responds with client ID and IPC handle.
+  EstablishGpuChannel() => (int32 client_id,

[Tom Sepez, 2016/08/25 17:14:24]

Whenever an ID is introduced, the question is "what happens when an untrusted
client hands back a spoofed one later on?"

[sadrul, 2016/08/25 22:03:13]

Hm, good point. I have reworked this so the client-id is an input parameter,
instead of an output parameter.

This GpuServiceInternal is a communication channel between the window server,
and the gpu process. i.e. untrusted clients cannot communicate to the gpu
process over this channel. The window-server is the one that assigns a
client-id, instead of the gpu process generating an id from here.
This is equivalent to the client-id currently used in the
GpuMsg_EstablishChannel in chrome:
https://cs.chromium.org/chromium/src/content/common/gpu_host_messages.h?sq=pa...).

+                            handle<message_pipe> channel_handle);
+};