Document confusing threading requirements in ServiceWorkerContext(Wrapper).

content/browser/service_worker/service_worker_context_wrapper.cc

 // Copyright 2013 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "content/browser/service_worker/service_worker_context_wrapper.h"
 
 #include <map>
 #include <set>
 #include <string>
 #include <utility>
@@ skipping 174 matching lines @@
         BrowserThread::IO,
         FROM_HERE,
         base::Bind(&ServiceWorkerContextWrapper::RegisterServiceWorker,
                    this,
                    pattern,
                    script_url,
                    continuation));
     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(
       net::SimplifyUrlForRequest(pattern),
       net::SimplifyUrlForRequest(script_url), NULL /* provider_host */,
       base::Bind(&FinishRegistrationOnIO, continuation));
 }
 
 static void FinishUnregistrationOnIO(
     const ServiceWorkerContext::ResultCallback& continuation,
@@ skipping 12 matching lines @@
     BrowserThread::PostTask(
         BrowserThread::IO,
         FROM_HERE,
         base::Bind(&ServiceWorkerContextWrapper::UnregisterServiceWorker,
                    this,
                    pattern,
                    continuation));
     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()->UnregisterServiceWorker(
       net::SimplifyUrlForRequest(pattern),
       base::Bind(&FinishUnregistrationOnIO, continuation));
 }
 
 void ServiceWorkerContextWrapper::UpdateRegistration(const GURL& pattern) {
   if (!BrowserThread::CurrentlyOn(BrowserThread::IO)) {
@@ skipping 457 matching lines @@
   observer_list_->Notify(FROM_HERE,
                          &ServiceWorkerContextObserver::OnStorageWiped);
 }
 
 ServiceWorkerContextCore* ServiceWorkerContextWrapper::context() {
   DCHECK_CURRENTLY_ON(BrowserThread::IO);
   return context_core_.get();
 }
 
 }  // namespace content