Unified: Relayout audio settings button on change

In order to show audio settings button on headphone connectivity change,
we should relayout the slider view when visibility is changed.

TEST=manual
BUG=880237

Change-Id: I966ee7082987ddbefb83a434657c5b733570ca94
Reviewed-on: https://chromium-review.googlesource.com/1215509Reviewed-by: Yoshiki Iguchi <yoshiki@chromium.org>
Commit-Queue: Tetsui Ohkubo <tetsui@chromium.org>
Cr-Commit-Position: refs/heads/master@{#590151}

ash/system/audio/unified_volume_view.cc

@@ -181,7 +181,7 @@ void UnifiedVolumeView::OnActiveInputNodeChanged() {
 }
 
 void UnifiedVolumeView::ChildVisibilityChanged(views::View* child) {
-  Update(true /* by_user */);
+  Layout();
 }
 
 }  // namespace ash