Use the correct guest icon in the new avatar bubble.

BUG=
R=oshima@chromium.org, pkasting@chromium.org

Review URL: https://codereview.chromium.org/59943005

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@233642 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 233642