Mus: Implement GpuMain mojo interface

services/ui/gpu/interfaces/BUILD.gn

 # 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.
 
 import("//mojo/public/tools/bindings/mojom.gni")
 
 mojom("interfaces") {
   sources = [
+    "gpu_main.mojom",
     "gpu_service_internal.mojom",
   ]
 
   public_deps = [
     "//gpu/ipc/common:interfaces",
     "//services/ui/public/interfaces",
   ]
 }