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
  virtual void OnURLFetchComplete(const URLFetcher* source,
                                  const GURL& url,
                                  const net::URLRequestStatus& status,
                                  int response_code,
                                  const net::ResponseCookies& cookies,
                                  const std::string& data) {
    running_ = (status.status() == net::URLRequestStatus::SUCCESS &&
                response_code == 200 && data.find("ok") == 0);
    MessageLoop::current()->Quit();
  }

  virtual void OnURLFetchComplete(const URLFetcher* source,
                                  const GURL& url,
                                  const net::URLRequestStatus& status,
                                  int response_code,
                                  const net::ResponseCookies& cookies,
                                  const std::string& data) {
    running_ = (status.status() == net::URLRequestStatus::SUCCESS &&
                response_code == 200 && data.find("ok") == 0);
    MessageLoop::current()->Quit();
  }
