Moves WmWindow::GetInternalWidget() into a standalone function

ash/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("//build/config/features.gni")
 import("//build/config/ui.gni")
 import("//testing/test.gni")
 import("//ui/base/ui_features.gni")
 
 # Historical note: Ash shipped on Windows for a couple years to support
@@ skipping 776 matching lines @@
     "wm/system_gesture_event_filter.h",
     "wm/system_modal_container_event_filter.cc",
     "wm/system_modal_container_event_filter.h",
     "wm/system_modal_container_event_filter_delegate.h",
     "wm/system_wallpaper_controller.cc",
     "wm/system_wallpaper_controller.h",
     "wm/toplevel_window_event_handler.cc",
     "wm/toplevel_window_event_handler.h",
     "wm/video_detector.cc",
     "wm/video_detector.h",
+    "wm/widget_finder.cc",
+    "wm/widget_finder.h",
     "wm/window_animations.cc",
     "wm/window_animations.h",
     "wm/window_cycle_event_filter_aura.cc",
     "wm/window_cycle_event_filter_aura.h",
     "wm/window_mirror_view.cc",
     "wm/window_mirror_view.h",
     "wm/window_properties.cc",
     "wm/window_properties.h",
     "wm/window_state_aura.cc",
     "wm/window_state_aura.h",
@@ skipping 682 matching lines @@
     "//content/public/app:both",
     "//device/bluetooth",
   ]
 
   data_deps = [
     "//ash/strings:ash_test_strings",
     "//ash/resources:ash_test_resources_with_content_100_percent",
     "//ash/resources:ash_test_resources_200_percent",
   ]
 }