Adds CaptureClientObserver as a way to track changes in capture

ui/aura/client/default_capture_client.cc

Index: ui/aura/client/default_capture_client.cc
diff --git a/ui/aura/client/default_capture_client.cc b/ui/aura/client/default_capture_client.cc
index b0719fc14e62849042298d330df840bcdd4f1c84..19f0d40c2a0127b1e21fa39d88db835983db2abd 100644
--- a/ui/aura/client/default_capture_client.cc
+++ b/ui/aura/client/default_capture_client.cc
@@ -4,27 +4,29 @@
 
 #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) {
@@ -44,6 +46,9 @@ void DefaultCaptureClient::SetCapture(Window* window) {
     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) {
@@ -60,5 +65,13 @@ 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