Http Cache: Add some debug code to track a bug. Basically

      just an early check for the headers.

BUG=56449
TEST=none


Review URL: http://codereview.chromium.org/3569025

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@61969 0039d316-1c4b-4281-b951-d872f2087c98