events: Update mouse-event handlers to not return EventResult.

ash/wm/user_activity_detector.cc

Index: ash/wm/user_activity_detector.cc
diff --git a/ash/wm/user_activity_detector.cc b/ash/wm/user_activity_detector.cc
index 6c55f896c4b3e5914a5459ab9d60311b07bc4c39..fccc6d8e7d01d7129752eb0eb0850cf431b7b588 100644
--- a/ash/wm/user_activity_detector.cc
+++ b/ash/wm/user_activity_detector.cc
@@ -38,13 +38,12 @@ void UserActivityDetector::OnKeyEvent(ui::KeyEvent* event) {
   MaybeNotify();
 }
 
-ui::EventResult UserActivityDetector::OnMouseEvent(ui::MouseEvent* event) {
+void UserActivityDetector::OnMouseEvent(ui::MouseEvent* event) {
   VLOG_IF(1, ignore_next_mouse_event_) << "ignoring mouse event";
   if (!(event->flags() & ui::EF_IS_SYNTHESIZED) &&
       !ignore_next_mouse_event_)
     MaybeNotify();
   ignore_next_mouse_event_ = false;
-  return ui::ER_UNHANDLED;
 }
 
 void UserActivityDetector::OnScrollEvent(ui::ScrollEvent* event) {