Ash Tray: Update message center position after display config changes.

Currently, only the quick settings bubble is updating its bounds when the
display configuration changes (e.g. screen rotation). We also need to update the
bounds of the message center as well.

TEST=manually on tablet device
BUG=1019506

Change-Id: I12e9d8f0ec92c5105f835b83a345e73f950baab8
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1894319Reviewed-by: Ahmed Mehfooz <amehfooz@chromium.org>
Commit-Queue: Ahmed Mehfooz <amehfooz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#712190}

ash/system/unified/unified_system_tray_bubble.cc

@@ -362,6 +362,9 @@ void UnifiedSystemTrayBubble::UpdateBubbleBounds() {
   bubble_view_->SetMaxHeight(max_height);
   bubble_view_->ChangeAnchorAlignment(tray_->shelf()->alignment());
   bubble_view_->ChangeAnchorRect(tray_->shelf()->GetSystemTrayAnchorRect());
+
+  if (tray_->IsMessageCenterBubbleShown())
+    tray_->message_center_bubble()->UpdatePosition();
 }
 
 void UnifiedSystemTrayBubble::CreateBlurLayerForAnimation() {