Make XHRs from extensions invisible to blocking webRequest API handlers

      This CL hides XmlHttpRequests from blocking webRequest API handlers that were triggered by the same extension that owns the handler. This is necessary because of a deadlock. Synchronous XHRs block the extension and prevent the handler from being executed.

BUG=105656
TEST=no


Review URL: http://codereview.chromium.org/8729028

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@112450 0039d316-1c4b-4281-b951-d872f2087c98