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

services/ui/ws/window_tree.cc

Index: services/ui/ws/window_tree.cc
diff --git a/services/ui/ws/window_tree.cc b/services/ui/ws/window_tree.cc
index 89ef41112b3c944243ac3b9822f10b336d9e6656..033b929a08a57240a012926e30fc7a3bcc154c5d 100644
--- a/services/ui/ws/window_tree.cc
+++ b/services/ui/ws/window_tree.cc
@@ -157,6 +157,14 @@ bool WindowTree::HasRoot(const ServerWindow* window) const {
   return roots_.count(window) > 0;
 }
 
+const ServerWindow* WindowTree::FindRootFor(const ServerWindow* window) const {
+  for (const ServerWindow* parent = window; parent; parent = parent->parent()) {
+    if (roots_.count(parent))
+      return parent;
+  }
+  return nullptr;
+}
+
 const ServerWindow* WindowTree::GetWindowByClientId(
     const ClientWindowId& id) const {
   auto iter = client_id_to_window_id_map_.find(id);