[Fetch API] Request's JS wrapper should be kept alive in FetchEvent

third_party/WebKit/LayoutTests/http/tests/serviceworker/chromium/resources/fetch-request-with-gc.js

+function wait(delay) {
+  return new Promise(resolve => setTimeout(resolve, delay));
+}
+
+self.addEventListener('fetch', e => {
+    self.gc();
+    e.respondWith(wait(10).then(() => {
+        self.gc();
+        return fetch(e.request);
+      }));
+  });