Change CaptureSynchronizer and PointerWatcherEventRouter to support multiple CaptureClients.

ui/aura/mus/window_tree_client.h

Index: ui/aura/mus/window_tree_client.h
diff --git a/ui/aura/mus/window_tree_client.h b/ui/aura/mus/window_tree_client.h
index f218b5e7e437d49cc1ad966a2c7100e5cb2d946b..6144da49f7f2e76a73bf83ef634abc10ce190dee 100644
--- a/ui/aura/mus/window_tree_client.h
+++ b/ui/aura/mus/window_tree_client.h
@@ -108,6 +108,9 @@ class AURA_EXPORT WindowTreeClient
 
   service_manager::Connector* connector() { return connector_; }
   ui::Gpu* gpu() { return gpu_.get(); }
+  CaptureSynchronizer* capture_synchronizer() {
+    return capture_synchronizer_.get();
+  }
 
   bool connected() const { return tree_ != nullptr; }
   ClientSpecificId client_id() const { return client_id_; }