CheckHasServiceWorkerCallback always called on the UI thread

This updates the code to match the documentation in
service_worker_context.h.

BUG=591427

Review-Url: https://codereview.chromium.org/2566623004
Cr-Commit-Position: refs/heads/master@{#437531}

content/browser/service_worker/service_worker_context_wrapper.cc

@@ -570,7 +570,7 @@ void ServiceWorkerContextWrapper::CheckHasServiceWorker(
     return;
   }
   if (!context_core_) {
-    BrowserThread::PostTask(BrowserThread::IO, FROM_HERE,
+    BrowserThread::PostTask(BrowserThread::UI, FROM_HERE,
                             base::Bind(callback, false));
     return;
   }