Change OnWindowInputEvent to use display_id to find the host and update event root_location in WS.

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 3ab40e1a744287a4446eb53f464d11aeb834519c..08c5c989ea04ec862e196c5821128c1f612771f8 100644
--- a/ui/aura/mus/window_tree_client.h
+++ b/ui/aura/mus/window_tree_client.h
@@ -478,6 +478,9 @@ class AURA_EXPORT WindowTreeClient
         cursor_location_mapping_.get());
   }
 
+  // Helper function to get the WindowTreeHostMus with the |display_id|.
+  WindowTreeHostMus* GetWindowTreeHostMusWithDisplayId(int64_t display_id);
+
   // This may be null in tests.
   service_manager::Connector* connector_;