Change CaptureSynchronizer and PointerWatcherEventRouter to support multiple CaptureClients.

ui/aura/mus/window_tree_client.cc

Index: ui/aura/mus/window_tree_client.cc
diff --git a/ui/aura/mus/window_tree_client.cc b/ui/aura/mus/window_tree_client.cc
index 5f73fbb14441b998bd131fb23754262ebcd3d516..90a74132bd1c1fc9ca77888cb11e3d37569f1e4e 100644
--- a/ui/aura/mus/window_tree_client.cc
+++ b/ui/aura/mus/window_tree_client.cc
@@ -467,8 +467,7 @@ void WindowTreeClient::WindowTreeConnectionEstablished(
   tree_ = window_tree;
 
   drag_drop_controller_ = base::MakeUnique<DragDropControllerMus>(this, tree_);
-  capture_synchronizer_ =
-      base::MakeUnique<CaptureSynchronizer>(this, tree_, GetCaptureClient());
+  capture_synchronizer_ = base::MakeUnique<CaptureSynchronizer>(this, tree_);
   focus_synchronizer_ = base::MakeUnique<FocusSynchronizer>(this, tree_);
 }