[Background Fetch] Cleanup/fix thread safety

content/browser/background_fetch/background_fetch_event_dispatcher.cc

Index: content/browser/background_fetch/background_fetch_event_dispatcher.cc
diff --git a/content/browser/background_fetch/background_fetch_event_dispatcher.cc b/content/browser/background_fetch/background_fetch_event_dispatcher.cc
index 291a19dc45ddaa35f17a40eab3c238d189c0eb9c..5a04c622c8918af14c54a8d24f5595ccf744e123 100644
--- a/content/browser/background_fetch/background_fetch_event_dispatcher.cc
+++ b/content/browser/background_fetch/background_fetch_event_dispatcher.cc
@@ -67,7 +67,9 @@ BackgroundFetchEventDispatcher::BackgroundFetchEventDispatcher(
     const scoped_refptr<ServiceWorkerContextWrapper>& service_worker_context)
     : service_worker_context_(service_worker_context) {}
 
-BackgroundFetchEventDispatcher::~BackgroundFetchEventDispatcher() = default;
+BackgroundFetchEventDispatcher::~BackgroundFetchEventDispatcher() {
+  DCHECK_CURRENTLY_ON(BrowserThread::IO);

[Peter Beverloo, 2017/07/10 13:13:21]

Please add a DCHECK_CURRENTLY_ON to the constructor as well, since that'll
happen on UI. Would be worthwhile to document its threading model in the header
file too.

[johnme, 2017/07/10 13:41:07]

Done here; the header already says "Must only be used on the IO thread."

+}
 
 void BackgroundFetchEventDispatcher::DispatchBackgroundFetchAbortEvent(
     const BackgroundFetchRegistrationId& registration_id,