Fix crash in ServiceWorkerDispatcherHost

This fixes the browser crash that happens when ServiceWorkerContextCore
has no live registration corresponding to the live version.
This is a quick fix intended to be mergeable.

BUG=459916

Review URL: https://codereview.chromium.org/982743002

Cr-Commit-Position: refs/heads/master@{#319214}

content/browser/service_worker/service_worker_dispatcher_host.cc

@@ -570,6 +570,9 @@ void ServiceWorkerDispatcherHost::OnSetHostedVersionId(
   ServiceWorkerRegistration* registration =
       GetContext()->GetLiveRegistration(version->registration_id());
   DCHECK(registration);
+  // TODO(ksakamoto): This is a quick fix for crbug.com/459916.
+  if (!registration)
+    return;
 
   // Set the document URL to the script url in order to allow
   // register/unregister/getRegistration on ServiceWorkerGlobalScope.