Use mus::MojoGpuMemoryBufferManager in mus::GpuService.

components/mus/public/cpp/BUILD.gn

 # 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.
 
 source_set("cpp") {
   sources = [
     "context_provider.h",
     "gles2_context.h",
     "input_event_handler.h",
     "lib/command_buffer_client_impl.cc",
     "lib/command_buffer_client_impl.h",
     "lib/context_provider.cc",
     "lib/gles2_context.cc",
-    "lib/gpu_memory_buffer_manager_mus.cc",
-    "lib/gpu_memory_buffer_manager_mus.h",
     "lib/gpu_service.cc",
     "lib/gpu_service.h",
     "lib/in_flight_change.cc",
     "lib/in_flight_change.h",
     "lib/output_surface.cc",
     "lib/property_type_converters.cc",
     "lib/scoped_window_ptr.cc",
     "lib/window.cc",
     "lib/window_observer.cc",
     "lib/window_private.cc",
@@ skipping 45 matching lines @@
     "//ui/events",
     "//ui/events/mojo",
     "//ui/gfx/geometry",
     "//ui/gfx/geometry/mojo",
   ]
 
   data_deps = [
     "//components/mus",
   ]
 }