Add ViewObserver to View for view updates

ui/views/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/ozone/ozone.gni")
 
 config("flags") {
@@ skipping 306 matching lines @@
     "style/platform_style.h",
     "style/platform_style_mac.mm",
     "view.cc",
     "view.h",
     "view_constants.cc",
     "view_constants.h",
     "view_model.cc",
     "view_model.h",
     "view_model_utils.cc",
     "view_model_utils.h",
+    "view_observer.h",
     "view_targeter.cc",
     "view_targeter.h",
     "view_targeter_delegate.cc",
     "view_targeter_delegate.h",
     "views_delegate.cc",
     "views_delegate.h",
     "views_export.h",
     "views_exports.cc",
     "views_switches.cc",
     "views_switches.h",
@@ skipping 677 matching lines @@
       "//ui/base:test_support",
       "//ui/compositor",
       "//ui/events:test_support",
       "//ui/gl:test_support",
       "//ui/resources",
       "//ui/resources:ui_test_pak",
       "//ui/strings",
     ]
   }
 }