Replace WmWindowObserver with aura::WindowObserver.

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 573 matching lines @@
     "common/wm_activation_observer.h",
     "common/wm_display_observer.h",
     "common/wm_layout_manager.h",
     "common/wm_lookup.cc",
     "common/wm_lookup.h",
     "common/wm_shell.cc",
     "common/wm_shell.h",
     "common/wm_transient_window_observer.h",
     "common/wm_window.cc",
     "common/wm_window.h",
-    "common/wm_window_observer.h",
     "common/wm_window_property.h",
-    "common/wm_window_tracker.h",
     "common/wm_window_user_data.h",
     "debug.cc",
     "debug.h",
     "default_wallpaper_delegate.cc",
     "default_wallpaper_delegate.h",
     "display/cursor_window_controller.cc",
     "display/cursor_window_controller.h",
     "display/display_animator.h",
     "display/display_animator_chromeos.cc",
     "display/display_animator_chromeos.h",
@@ skipping 804 matching lines @@
     "//content/public/app:both",
     "//device/bluetooth",
   ]
 
   data_deps = [
     "//ash/common/strings:ash_test_strings",
     "//ash/resources:ash_test_resources_with_content_100_percent",
     "//ash/resources:ash_test_resources_200_percent",
   ]
 }