Path not specified in data reduction proxy precomputed credentials

When the path is not specified, the search for a auth cache entry
fails, and a fetch must resort to handling a 407.

BUG=329633

Review URL: https://codereview.chromium.org/110373006

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@241946 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 241946