refactor: follow upstream's suggestions on when to use EmptyGURL() (#45521)

- Prefer GURL() when we want to return a non-reference empty URL.

- In ServiceWorkerMain::GetStorageKey(), use a reference instead
  of instantiating a new temporary GURL.

From url/gurl.h:

> // Returns a reference to a singleton empty GURL. This object is for
> // callers who return references but don't have anything to return in
> // some cases. If you just want an empty URL for normal use, prefer
> // GURL().

Co-authored-by: trop[bot] <37223003+trop[bot]@users.noreply.github.com>
Co-authored-by: Charles Kerr <charles@charleskerr.com>

shell/browser/api/electron_api_web_frame_main.cc

@@ -375,7 +375,7 @@ int WebFrameMain::RoutingID() const {
 
 GURL WebFrameMain::URL() const {
   if (!CheckRenderFrame())
-    return GURL::EmptyGURL();
+    return {};
   return render_frame_->GetLastCommittedURL();
 }