Remove incorrect thread restriction on ExtensionSWMessageFilter dtor.

ExtensionServiceWorkerMessageFilter is constructed on UI and it
*often* gets destroyed in IO thread, but there is no such gurantee.
It is possible for it to be destroyed on UI thread as it also handles
handful of messages on UI thread.

Remove the IO thread restriction, also add DCHECK in constructor.

Bug: None
Change-Id: I15ec9ecd347c44a8fc8fdea031f72b27689190b2
Reviewed-on: https://chromium-review.googlesource.com/1231695Reviewed-by: Karan Bhatia <karandeepb@chromium.org>
Commit-Queue: Istiaque Ahmed <lazyboy@chromium.org>
Cr-Commit-Position: refs/heads/master@{#592218}

extensions/browser/extension_service_worker_message_filter.cc

@@ -22,12 +22,12 @@ ExtensionServiceWorkerMessageFilter::ExtensionServiceWorkerMessageFilter(
       browser_context_(context),
       render_process_id_(render_process_id),
       service_worker_context_(service_worker_context),
-      dispatcher_(new ExtensionFunctionDispatcher(context)) {}
-
-ExtensionServiceWorkerMessageFilter::~ExtensionServiceWorkerMessageFilter() {
-  DCHECK_CURRENTLY_ON(content::BrowserThread::IO);
+      dispatcher_(new ExtensionFunctionDispatcher(context)) {
+  DCHECK_CURRENTLY_ON(content::BrowserThread::UI);
 }
 
+ExtensionServiceWorkerMessageFilter::~ExtensionServiceWorkerMessageFilter() {}
+
 void ExtensionServiceWorkerMessageFilter::OverrideThreadForMessage(
     const IPC::Message& message,
     content::BrowserThread::ID* thread) {