Add a ServiceWorkerContextObserver to content's public API.

content/public/browser/service_worker_context_observer.h

+// Copyright 2017 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#ifndef CONTENT_PUBLIC_BROWSER_SERVICE_WORKER_CONTEXT_OBSERVER_H_
+#define CONTENT_PUBLIC_BROWSER_SERVICE_WORKER_CONTEXT_OBSERVER_H_
+
+#include "url/gurl.h"
+
+namespace content {
+
+class ServiceWorkerContextObserver {
+ public:
+  // Called when a service worker has been registered with scope |pattern|.
+  virtual void OnRegistrationStored(const GURL& pattern) {}
+
+ protected:
+  virtual ~ServiceWorkerContextObserver() {}
+};
+
+}  // namespace content
+
+#endif  // CONTENT_PUBLIC_BROWSER_SERVICE_WORKER_CONTEXT_OBSERVER_H_