Adds CaptureClientObserver as a way to track changes in capture

ui/wm/core/capture_controller.cc

Index: ui/wm/core/capture_controller.cc
diff --git a/ui/wm/core/capture_controller.cc b/ui/wm/core/capture_controller.cc
index b10cc1a76badcb279e15647d900877f8b1ad14b8..08238de931378184056d7b4e2184a40a227ee008 100644
--- a/ui/wm/core/capture_controller.cc
+++ b/ui/wm/core/capture_controller.cc
@@ -4,6 +4,7 @@
 
 #include "ui/wm/core/capture_controller.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_tracker.h"
@@ -72,6 +73,9 @@ void CaptureController::SetCapture(aura::Window* new_capture_window) {
     if (capture_delegate_)
       capture_delegate_->SetNativeCapture();
   }
+
+  for (aura::client::CaptureClientObserver& observer : observers_)
+    observer.OnCaptureChanged(old_capture_window, capture_window_);
 }
 
 void CaptureController::ReleaseCapture(aura::Window* window) {
@@ -88,6 +92,16 @@ aura::Window* CaptureController::GetGlobalCaptureWindow() {
   return capture_window_;
 }
 
+void CaptureController::AddObserver(
+    aura::client::CaptureClientObserver* observer) {
+  observers_.AddObserver(observer);
+}
+
+void CaptureController::RemoveObserver(
+    aura::client::CaptureClientObserver* observer) {
+  observers_.RemoveObserver(observer);
+}
+
 ////////////////////////////////////////////////////////////////////////////////
 // CaptureController, private: