views/mus: Move PlatformWindowMus into its own file.

ui/views/mus/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.
 
 import("//tools/grit/repack.gni")
 
 source_set("mus") {
   sources = [
     "aura_init.cc",
     "aura_init.h",
     "display_converter.cc",
     "display_converter.h",
     "input_method_mus.cc",
     "input_method_mus.h",
     "native_widget_mus.cc",
     "native_widget_mus.h",
+    "platform_window_mus.cc",
+    "platform_window_mus.h",
     "surface_binding.cc",
     "surface_binding.h",
     "surface_context_factory.cc",
     "surface_context_factory.h",
     "window_manager_client_area_insets.h",
     "window_manager_connection.cc",
     "window_manager_connection.h",
     "window_tree_host_mus.cc",
     "window_tree_host_mus.h",
   ]
@@ skipping 54 matching lines @@
     "$root_gen_dir/ui/strings/ui_strings_en-US.pak",
     "$root_gen_dir/ui/views/resources/views_resources_100_percent.pak",
   ]
   output = "$root_out_dir/views_mus_resources.pak"
   deps = [
     "//ui/strings",
     "//ui/resources",
     "//ui/views/resources",
   ]
 }