app_list: Clear start page contents on NOTIFICATION_PROFILE_DESTROYED

NOTIFICATION_APP_TERMINATING could happen before closing app list bubble
on shutdown and breaks the assumption that start page contents should
out-live app list.

BUG=314567
R=mukai@chromium.org

Review URL: https://codereview.chromium.org/57943005

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@233015 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 233015