Document confusing threading requirements in ServiceWorkerContext(Wrapper).

content/browser/service_worker/service_worker_context_wrapper.cc

Index: content/browser/service_worker/service_worker_context_wrapper.cc
diff --git a/content/browser/service_worker/service_worker_context_wrapper.cc b/content/browser/service_worker/service_worker_context_wrapper.cc
index 280cc29bffa4727febeaed83c7a677891ac74cce..85d9830a529c493123c99ac40774c86816ed4696 100644
--- a/content/browser/service_worker/service_worker_context_wrapper.cc
+++ b/content/browser/service_worker/service_worker_context_wrapper.cc
@@ -192,10 +192,8 @@ void ServiceWorkerContextWrapper::RegisterServiceWorker(
     return;
   }
   if (!context_core_) {
-    BrowserThread::PostTask(
-        BrowserThread::IO,
-        FROM_HERE,
-        base::Bind(continuation, false));
+    BrowserThread::PostTask(BrowserThread::UI, FROM_HERE,
+                            base::Bind(continuation, false));
     return;
   }
   context()->RegisterServiceWorker(
@@ -228,10 +226,8 @@ void ServiceWorkerContextWrapper::UnregisterServiceWorker(
     return;
   }
   if (!context_core_) {
-    BrowserThread::PostTask(
-        BrowserThread::IO,
-        FROM_HERE,
-        base::Bind(continuation, false));
+    BrowserThread::PostTask(BrowserThread::UI, FROM_HERE,
+                            base::Bind(continuation, false));
     return;
   }