Change CaptureSynchronizer and PointerWatcherEventRouter to support multiple CaptureClients.

ui/views/mus/pointer_watcher_event_router.h

Index: ui/views/mus/pointer_watcher_event_router.h
diff --git a/ui/views/mus/pointer_watcher_event_router.h b/ui/views/mus/pointer_watcher_event_router.h
index 7414097f04b5ef32005d87cf76a81deb41767f7c..4298b7816e6101fd7f883cff289d02e547fb2aa5 100644
--- a/ui/views/mus/pointer_watcher_event_router.h
+++ b/ui/views/mus/pointer_watcher_event_router.h
@@ -14,6 +14,10 @@
 
 namespace aura {
 class WindowTreeClient;
+
+namespace client {
+class CaptureClient;
+}
 }
 
 namespace ui {
@@ -56,6 +60,10 @@ class VIEWS_MUS_EXPORT PointerWatcherEventRouter
   void OnPointerEventObserved(const ui::PointerEvent& event,
                               aura::Window* target);
 
+  // Called when the |capture_client| has been set or will be unset.
+  void AttachToCaptureClient(aura::client::CaptureClient* capture_client);
+  void DetachFromCaptureClient(aura::client::CaptureClient* capture_client);
+
  private:
   friend class PointerWatcherEventRouterTest;