bindings: Fix how to check a member is missing

IDL dictionary provides a way to check if a member is missing or not.
This CL fixes a wrong way of the check in ServiceWorkerContainer.

Bug: 839389
Change-Id: I7134bbcdfb290db26fca14aeaa903d165215097d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2224421
Commit-Queue: Hitoshi Yoshida <peria@chromium.org>
Commit-Queue: Kentaro Hara <haraken@chromium.org>
Auto-Submit: Hitoshi Yoshida <peria@chromium.org>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Cr-Commit-Position: refs/heads/master@{#774445}

third_party/blink/renderer/modules/service_worker/service_worker_container.cc

@@ -280,10 +280,10 @@ ScriptPromise ServiceWorkerContainer::registerServiceWorker(
   }
 
   KURL scope_url;
-  if (options->scope().IsNull())
-    scope_url = KURL(script_url, "./");
-  else
+  if (options->hasScope())
     scope_url = execution_context->CompleteURL(options->scope());
+  else
+    scope_url = KURL(script_url, "./");
   scope_url.RemoveFragmentIdentifier();
 
   if (!SchemeRegistry::ShouldTreatURLSchemeAsAllowingServiceWorkers(