Adds CaptureClientObserver as a way to track changes in capture

ui/aura/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/ui.gni")
 import("//testing/test.gni")
 
 component("aura") {
   sources = [
     "../wm/public/activation_change_observer.cc",
@@ skipping 16 matching lines @@
     "../wm/public/tooltip_client.h",
     "../wm/public/transient_window_client.cc",
     "../wm/public/transient_window_client.h",
     "../wm/public/window_move_client.cc",
     "../wm/public/window_move_client.h",
     "../wm/public/window_types.h",
     "client/aura_constants.cc",
     "client/aura_constants.h",
     "client/capture_client.cc",
     "client/capture_client.h",
+    "client/capture_client_observer.h",
     "client/capture_delegate.h",
     "client/cursor_client.cc",
     "client/cursor_client.h",
     "client/cursor_client_observer.cc",
     "client/cursor_client_observer.h",
     "client/default_capture_client.cc",
     "client/default_capture_client.h",
     "client/event_client.cc",
     "client/event_client.h",
     "client/focus_change_observer.cc",
@@ skipping 235 matching lines @@
   ]
 
   if (is_linux) {
     deps += [ "//third_party/mesa" ]
   }
 
   data_deps = [
     "//third_party/mesa:osmesa",
   ]
 }