Fix assertion.

This code was the result of a review comment I forgot to test before checking in.

BUG=
R=scottmg@chromium.org

Review URL: https://codereview.chromium.org/27757002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@229220 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 229220