Adds CaptureClientObserver as a way to track changes in capture

ui/aura/client/capture_client.h

Index: ui/aura/client/capture_client.h
diff --git a/ui/aura/client/capture_client.h b/ui/aura/client/capture_client.h
index a792fb377a11c227b1b560be69fb1bd17689cdce..c7874267e8b0645d48631d97b7067f6407616109 100644
--- a/ui/aura/client/capture_client.h
+++ b/ui/aura/client/capture_client.h
@@ -12,6 +12,8 @@ class Window;
 
 namespace client {
 
+class CaptureClientObserver;
+
 // An interface implemented by an object that manages input capture.
 class AURA_EXPORT CaptureClient {
  public:
@@ -31,6 +33,9 @@ class AURA_EXPORT CaptureClient {
   // See description of GetCaptureWindow() for details.
   virtual Window* GetGlobalCaptureWindow() = 0;
 
+  virtual void AddObserver(CaptureClientObserver* observer) = 0;
+  virtual void RemoveObserver(CaptureClientObserver* observer) = 0;
+
  protected:
   virtual ~CaptureClient() {}
 };