Adds CaptureClientObserver as a way to track changes in capture

ui/aura/client/default_capture_client.cc

 // Copyright (c) 2012 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.
 
 #include "ui/aura/client/default_capture_client.h"
 
+#include "ui/aura/client/capture_client_observer.h"
 #include "ui/aura/window.h"
 #include "ui/aura/window_event_dispatcher.h"
 #include "ui/aura/window_tree_host.h"
 
 namespace aura {
 namespace client {
+namespace {
 
-// static
 // Track the active capture window across root windows.
 Window* global_capture_window_ = nullptr;
 
+}  // namespace
+
 DefaultCaptureClient::DefaultCaptureClient(Window* root_window)
-    : root_window_(root_window),
-      capture_window_(NULL) {
+    : root_window_(root_window), capture_window_(nullptr) {
   SetCaptureClient(root_window_, this);
 }
 
 DefaultCaptureClient::~DefaultCaptureClient() {
   if (global_capture_window_ == capture_window_)
     global_capture_window_ = nullptr;
-  SetCaptureClient(root_window_, NULL);
+  SetCaptureClient(root_window_, nullptr);
 }
 
 void DefaultCaptureClient::SetCapture(Window* window) {
   if (capture_window_ == window)
     return;
   if (window)
     ui::GestureRecognizer::Get()->CancelActiveTouchesExcept(window);
 
   Window* old_capture_window = capture_window_;
   capture_window_ = window;
   global_capture_window_ = window;
 
   CaptureDelegate* capture_delegate = root_window_->GetHost()->dispatcher();
   if (capture_window_)
     capture_delegate->SetNativeCapture();
   else
     capture_delegate->ReleaseNativeCapture();
 
   capture_delegate->UpdateCapture(old_capture_window, capture_window_);
+
+  for (CaptureClientObserver& observer : observers_)
+    observer.OnCaptureChanged(old_capture_window, capture_window_);
 }
 
 void DefaultCaptureClient::ReleaseCapture(Window* window) {
   if (capture_window_ != window)
     return;
   SetCapture(NULL);
 }
 
 Window* DefaultCaptureClient::GetCaptureWindow() {
   return capture_window_;
 }
 
 Window* DefaultCaptureClient::GetGlobalCaptureWindow() {
   return global_capture_window_;
 }
 
+void DefaultCaptureClient::AddObserver(CaptureClientObserver* observer) {
+  observers_.AddObserver(observer);
+}
+
+void DefaultCaptureClient::RemoveObserver(CaptureClientObserver* observer) {
+  observers_.RemoveObserver(observer);
+}
+
 }  // namespace client
 }  // namespace aura