Enable V3 fetcher mode for privet v3 session.

Missed from https://codereview.chromium.org/454623002

Review URL: https://codereview.chromium.org/618453003

Cr-Commit-Position: refs/heads/master@{#297322}

chrome/browser/local_discovery/privetv3_session.cc

@@ -133,6 +133,7 @@ void PrivetV3Session::StartRequest(Request* request) {
   url_fetcher->SetUploadData(cloud_print::kContentTypeJSON, json);
 
   request->fetcher_delegate_->url_fetcher_ = url_fetcher.Pass();
+  request->fetcher_delegate_->url_fetcher_->V3Mode();
   request->fetcher_delegate_->url_fetcher_->Start();
 }