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 230 matching lines @@
     "controls/textfield/textfield_model.cc",
     "controls/textfield/textfield_model.h",
     "controls/throbber.cc",
     "controls/throbber.h",
     "controls/tree/tree_view.cc",
     "controls/tree/tree_view.h",
     "controls/tree/tree_view_controller.cc",
     "controls/tree/tree_view_controller.h",
     "debug_utils.cc",
     "debug_utils.h",
+    "devtools_observer.h",
     "drag_controller.h",
     "drag_utils.cc",
     "drag_utils.h",
     "drag_utils_mac.mm",
     "event_monitor.h",
     "event_monitor_mac.h",
     "event_monitor_mac.mm",
     "focus/external_focus_tracker.cc",
     "focus/external_focus_tracker.h",
     "focus/focus_manager.cc",
@@ skipping 757 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",
     ]
   }
 }