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 13660c96943379fbde45e8063fb00f656942a44e..ec0998f61b8ce267ebc3e88d73bd9ecce49447a5 100644
--- a/ui/aura/mus/window_tree_client.cc
+++ b/ui/aura/mus/window_tree_client.cc
@@ -453,8 +453,7 @@ 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_);
 }
 
@@ -1639,6 +1638,7 @@ void WindowTreeClient::OnWindowTreeHostCreated(
   // the DragDropClient.
   client::SetDragDropClient(window_tree_host->window(),
                             drag_drop_controller_.get());
+  window_tree_host->window()->AddObserver(capture_synchronizer_.get());
 }
 
 void WindowTreeClient::OnTransientChildWindowAdded(Window* parent,