Adds a basic offline check to InstallableManager.

chrome/test/data/banners/service_worker.js

Index: chrome/test/data/banners/service_worker.js
diff --git a/chrome/test/data/banners/service_worker.js b/chrome/test/data/banners/service_worker.js
index 1047484ea8ad0c9f6964dc0d605c651c160dbdb7..22d5efe8ff5f4ff1e170188822f37c38a49b797c 100644
--- a/chrome/test/data/banners/service_worker.js
+++ b/chrome/test/data/banners/service_worker.js
@@ -2,4 +2,7 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-self.addEventListener('activate', function(event) {});
+self.addEventListener('activate', function(event) {});
+
+// A no-op fetch handler.
+self.addEventListener('fetch', e => e.respondWith(fetch(e.request)));