Change CaptureSynchronizer and PointerWatcherEventRouter to support multiple CaptureClients.

ui/views/mus/mus_client.h

Index: ui/views/mus/mus_client.h
diff --git a/ui/views/mus/mus_client.h b/ui/views/mus/mus_client.h
index 72792a72c520077a42ff6d760307bdc04bb16de2..e799d4734b6a04b9b3bff65e373bacd23afc991d 100644
--- a/ui/views/mus/mus_client.h
+++ b/ui/views/mus/mus_client.h
@@ -12,6 +12,7 @@
 #include <vector>
 
 #include "base/macros.h"
+#include "ui/aura/client/capture_client.h"
 #include "ui/aura/mus/window_tree_client_delegate.h"
 #include "ui/base/dragdrop/os_exchange_data_provider_factory.h"
 #include "ui/views/mus/mus_export.h"
@@ -101,6 +102,14 @@ class VIEWS_MUS_EXPORT MusClient
   NativeWidget* CreateNativeWidget(const Widget::InitParams& init_params,
                                    internal::NativeWidgetDelegate* delegate);
 
+  // Called when the capture client has been set for a window to notify
+  // PointerWatcherEventRouter and CaptureSynchronizer.
+  void OnCaptureClientSet(aura::client::CaptureClient* capture_client);
+
+  // Called when the capture client will be unset for a window to notify
+  // PointerWatcherEventRouter and CaptureSynchronizer.
+  void OnCaptureClientUnset(aura::client::CaptureClient* capture_client);
+
   void AddObserver(MusClientObserver* observer);
   void RemoveObserver(MusClientObserver* observer);