Fix crash when a (?) tooltip bubble is popped by the sign in webview.

R=estade@chromium.org
BUG=329928

Review URL: https://codereview.chromium.org/114613005

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@242011 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 242011