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

ui/aura/mus/window_tree_client.h

 // Copyright 2014 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef UI_AURA_MUS_WINDOW_TREE_CLIENT_H_
 #define UI_AURA_MUS_WINDOW_TREE_CLIENT_H_
 
 #include <stdint.h>
 
 #include <map>
@@ skipping 460 matching lines @@
   // Overrided from FocusSynchronizerDelegate:
   uint32_t CreateChangeIdForFocus(WindowMus* window) override;
 
   // The one int in |cursor_location_mapping_|. When we read from this
   // location, we must always read from it atomically.
   base::subtle::Atomic32* cursor_location_memory() {
     return reinterpret_cast<base::subtle::Atomic32*>(
         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_;
 
   // This is set once and only once when we get OnEmbed(). It gives the unique
   // id for this client.
   ClientSpecificId client_id_;
 
   // Id assigned to the next window created.
   ClientSpecificId next_window_id_;
 
@@ skipping 55 matching lines @@
   std::unique_ptr<ui::Gpu> gpu_;
   std::unique_ptr<MusContextFactory> compositor_context_factory_;
   base::WeakPtrFactory<WindowTreeClient> weak_factory_;
 
   DISALLOW_COPY_AND_ASSIGN(WindowTreeClient);
 };
 
 }  // namespace aura
 
 #endif  // UI_AURA_MUS_WINDOW_TREE_CLIENT_H_