Remove helper() from FakeServiceWorker

This doesn't seem to be used.

Bug: N/A
Change-Id: I050398b164cb58d86ee7b1af271fa91fdc43a2ec
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2294500Reviewed-by: Matt Falkenhagen <falken@chromium.org>
Commit-Queue: Kenichi Ishibashi <bashi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#787643}

content/browser/service_worker/fake_service_worker.h

@@ -35,8 +35,6 @@ class FakeServiceWorker : public blink::mojom::ServiceWorker {
     return host_;
   }
 
-  EmbeddedWorkerTestHelper* helper() { return helper_; }
-
   void Bind(mojo::PendingReceiver<blink::mojom::ServiceWorker> receiver);
 
   // Returns after InitializeGlobalScope() is called.