[CrOS PhoneHub] Fix bubble size not fit after state transitions.

This CL fixes the issue where the bubble size doesn't get updated after
switching to another content view possibly with a different size.

Screenshot before this CL:
https://screenshot.googleplex.com/4GN26KQ6QC6jyo4.png

BUG=1106937,1126208

Change-Id: Ie6dda62d93f4d1e5c09212426e8b55bfc5d30cbd
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2446773Reviewed-by: Tim Song <tengs@chromium.org>
Reviewed-by: Meilin Wang <meilinw@chromium.org>
Commit-Queue: Meilin Wang <meilinw@chromium.org>
Cr-Commit-Position: refs/heads/master@{#813428}

ash/system/phonehub/phone_hub_tray.cc

@@ -119,6 +119,9 @@ void PhoneHubTray::OnPhoneHubUiStateChanged() {
     bubble_view->RemoveChildView(content_view_);
   content_view_ = content_view.get();
   bubble_view->AddChildView(std::move(content_view));
+
+  // Updates bubble to handle possible size change with a different child view.
+  bubble_view->UpdateBubble();
 }
 
 void PhoneHubTray::AnchorUpdated() {