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

services/ui/ws/test_change_tracker.cc

Index: services/ui/ws/test_change_tracker.cc
diff --git a/services/ui/ws/test_change_tracker.cc b/services/ui/ws/test_change_tracker.cc
index 172c9b81a49de01b22d4f914651b16a2bbcb992f..3bb89afcc2b3fcd434706f1e503b7ee0525ba2c2 100644
--- a/services/ui/ws/test_change_tracker.cc
+++ b/services/ui/ws/test_change_tracker.cc
@@ -106,6 +106,11 @@ std::string ChangeToDescription(const Change& change,
           WindowIdToString(change.window_id).c_str(), change.event_action);
       if (change.matches_pointer_watcher)
         result += " matches_pointer_watcher";
+      if (!change.event_root_location.IsOrigin()) {
+        result +=
+            base::StringPrintf(" event_root_location=%s",
+                               change.event_root_location.ToString().c_str());
+      }
       return result;
     }
 
@@ -365,6 +370,8 @@ void TestChangeTracker::OnWindowInputEvent(Id window_id,
   change.window_id = window_id;
   change.event_action = static_cast<int32_t>(event.type());
   change.matches_pointer_watcher = matches_pointer_watcher;
+  if (event.IsLocatedEvent())
+    change.event_root_location = event.AsLocatedEvent()->root_location();
   AddChange(change);
 }