Fix test_shell to use the correct page ID for error pages.

      BUG=70315
TEST=Go back to a &quotconfirm form resubmission&quot error, then forward.

Review URL: http://codereview.chromium.org/6286100

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@73827 0039d316-1c4b-4281-b951-d872f2087c98