Adds CaptureClientObserver as a way to track changes in capture

ui/aura/client/default_capture_client.h

Index: ui/aura/client/default_capture_client.h
diff --git a/ui/aura/client/default_capture_client.h b/ui/aura/client/default_capture_client.h
index d9dbd57818036abdaf77958357335170b96b71fe..40ab823b9ea25bdb1deb85bca8a2e38497fc7ff4 100644
--- a/ui/aura/client/default_capture_client.h
+++ b/ui/aura/client/default_capture_client.h
@@ -5,8 +5,8 @@
 #ifndef UI_AURA_CLIENT_DEFAULT_CAPTURE_CLIENT_H_
 #define UI_AURA_CLIENT_DEFAULT_CAPTURE_CLIENT_H_
 
-#include "base/compiler_specific.h"
 #include "base/macros.h"
+#include "base/observer_list.h"
 #include "ui/aura/aura_export.h"
 #include "ui/aura/client/capture_client.h"
 
@@ -15,7 +15,7 @@ namespace client {
 
 class AURA_EXPORT DefaultCaptureClient : public client::CaptureClient {
  public:
-  explicit DefaultCaptureClient(Window* root_window);
+  explicit DefaultCaptureClient(Window* root_window = nullptr);
   ~DefaultCaptureClient() override;
 
  protected:
@@ -24,10 +24,13 @@ class AURA_EXPORT DefaultCaptureClient : public client::CaptureClient {
   void ReleaseCapture(Window* window) override;
   Window* GetCaptureWindow() override;
   Window* GetGlobalCaptureWindow() override;
+  void AddObserver(CaptureClientObserver* observer) override;
+  void RemoveObserver(CaptureClientObserver* observer) override;
 
  private:
-  Window* root_window_;
+  Window* root_window_;  // May be null.
   Window* capture_window_;
+  base::ObserverList<CaptureClientObserver> observers_;
 
   DISALLOW_COPY_AND_ASSIGN(DefaultCaptureClient);
 };