Block reading unauthenticated bytes from the proxy when the user cancels a 407 proxy auth request while we're trying to establish an SSL tunnel.

      BUG=8473
R=wtc

Review URL: http://codereview.chromium.org/42065

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@11484 0039d316-1c4b-4281-b951-d872f2087c98