rAc: fix another crash involving bubbles and dead widgets :(

BUG=330308
TEST=no crashes
R=estade@chromium.org

Review URL: https://codereview.chromium.org/114333005

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@242268 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 242268