Chromecast: moves ConnectivityChecker to the IO thread.

R=derekjchow@chromium.org,byungchul@chromium.org,lcwu@chromium.org
BUG=476713

Review URL: https://codereview.chromium.org/1058143004

Cr-Commit-Position: refs/heads/master@{#325446}

chromecast/browser/cast_browser_main_parts.cc

@@ -270,7 +270,7 @@ void CastBrowserMainParts::PreMainMessageLoopRun() {
   cast_browser_process_->SetConnectivityChecker(
       make_scoped_refptr(new ConnectivityChecker(
           content::BrowserThread::GetMessageLoopProxyForThread(
-              content::BrowserThread::FILE))));
+              content::BrowserThread::IO))));
 
   url_request_context_factory_->InitializeOnUIThread();