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

services/ui/ws/window_tree.h

Index: services/ui/ws/window_tree.h
diff --git a/services/ui/ws/window_tree.h b/services/ui/ws/window_tree.h
index 7bb3106f2b93db999857d127d59c3ce754f77ec5..49299fccce064bcdf378e8c1121e6f6a0979df5c 100644
--- a/services/ui/ws/window_tree.h
+++ b/services/ui/ws/window_tree.h
@@ -118,6 +118,9 @@ class WindowTree : public mojom::WindowTree,
   // Returns true if |window| is one of this trees roots.
   bool HasRoot(const ServerWindow* window) const;
 
+  // Find the correct root for |window| in this tree's roots.
+  const ServerWindow* FindRootFor(const ServerWindow* window) const;
+
   std::set<const ServerWindow*> roots() { return roots_; }
 
   void set_name(const std::string& name) { name_ = name; }