Bocking onReceivedHeaders signal for webRequest API

      This is a new signal that allows extensions to
- Modify/Delete cookies of HTTP responses before they are seen by the cookie monster.
- Cancel requests based on the length or content type of the HTTP response.

BUG=89118
TEST=no


Review URL: http://codereview.chromium.org/7931026

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@104091 0039d316-1c4b-4281-b951-d872f2087c98