Use URLFetcher::Create instead of new in http_bridge.cc.

 This change modified http_bridge so that it uses a factory to construct
the URLFetcher. Moreover, it modified sync_backend_host_unittest.cc to
use an URLFetcher factory which will prevent access to www.example.com during
the test.

BUG=none
TEST=sync_backend_host_unittest.cc

Review URL: http://codereview.chromium.org/7053011

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@87227 0039d316-1c4b-4281-b951-d872f2087c98
  void RunDelegate() {
    net::URLRequestStatus status;
    status.set_status(success_ ? net::URLRequestStatus::SUCCESS :
                                 net::URLRequestStatus::FAILED);
    delegate()->OnURLFetchComplete(this, url_, status, success_ ? 200 : 500,
                                   net::ResponseCookies(), response_data_);
  }

  void RunDelegate() {
    net::URLRequestStatus status;
    status.set_status(success_ ? net::URLRequestStatus::SUCCESS :
                                 net::URLRequestStatus::FAILED);
    delegate()->OnURLFetchComplete(this, url_, status, success_ ? 200 : 500,
                                   net::ResponseCookies(), response_data_);
  }
