Ash Tray: Fix ARC pop-up notifications not being auto dismissed.

Currently, ARC pop-ups stay around forever because the dismiss timer is paused
when hovering on the notification, but mouse exit events are not properly
dispatched to the widget, so the timer is never restarted.

TEST=manual
BUG=995567

Change-Id: Id2892fda60f2c170abddf5c812e23150c61c3d47
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1783701
Reviewed-by: Ahmed Mehfooz <amehfooz@chromium.org>
Commit-Queue: Tim Song <tengs@chromium.org>
Cr-Commit-Position: refs/heads/master@{#693913}

ash/system/message_center/arc/arc_notification_content_view.cc

@@ -94,6 +94,7 @@ class ArcNotificationContentView::EventForwarder : public ui::EventHandler {
       if (located_event->type() == ui::ET_MOUSE_ENTERED ||
           located_event->type() == ui::ET_MOUSE_EXITED) {
         owner_->UpdateControlButtonsVisibility();
+        widget->OnMouseEvent(located_event->AsMouseEvent());
         return;
       }