[ash] Segfault when shelf position is changed by app's context menu in the overflow bubble

When user changes shelf position to different one from current settings,
overflow bubble is destroyed before calling OverflowBubble::HideBubbleAndRefreshButton().

R=jamescook@chromium.org
BUG=305518
TEST=visual test

Review URL: https://codereview.chromium.org/26620002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@227896 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 227896