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
HttpBridge::RequestContextGetter::GetURLRequestContext() {
  if (!context_) {
    net::URLRequestContext* baseline_context =
        baseline_context_getter_->GetURLRequestContext();
    context_ = new RequestContext(baseline_context);
    baseline_context_getter_ = NULL;
  }

  if (is_user_agent_set())
    context_->set_user_agent(user_agent_);

  return context_;
}

HttpBridge::RequestContextGetter::GetURLRequestContext() {
  if (!context_) {
    net::URLRequestContext* baseline_context =
        baseline_context_getter_->GetURLRequestContext();
    context_ = new RequestContext(baseline_context);
    baseline_context_getter_ = NULL;
  }

  if (is_user_agent_set())
    context_->set_user_agent(user_agent_);

  return context_;
}
