[ServiceWorker] Avoid using null thread local pointer while exiting worker thread

ServiceWorkerGlobalScope#registration (WebServiceWorkerRegistrationImpl)
can still be alive and handle incoming messages even the thread local
pointer ServiceWorkerContextClient::ThreadSpecificInstance() has already
been reset to nullptr by
ServiceWorkerContextClient::WillDestroyWorkerContext(), thus, we need to
add a guard logic into WebServiceWorkerRegistrationImpl to avoid
accessing nullptr.

BUG=852195

Change-Id: Ia07b88f40d195293db8690216bd46d1a22bd687a
Reviewed-on: https://chromium-review.googlesource.com/1098563Reviewed-by: Makoto Shimazu <shimazu@chromium.org>
Commit-Queue: Han Leon <leon.han@intel.com>
Cr-Commit-Position: refs/heads/master@{#567178}

content/renderer/service_worker/web_service_worker_registration_impl.cc

@@ -314,7 +314,7 @@ WebServiceWorkerRegistrationImpl::GetOrCreateServiceWorkerObject(
           provider_context_for_client_->GetOrCreateServiceWorkerObject(
               std::move(info));
     }
-  } else {
+  } else if (ServiceWorkerContextClient::ThreadSpecificInstance()) {
     service_worker = ServiceWorkerContextClient::ThreadSpecificInstance()
                          ->GetOrCreateServiceWorkerObject(std::move(info));
   }