CREDENTIAL: Rework the integration with Fetch (2/2)

third_party/WebKit/LayoutTests/http/tests/credentialmanager/resources/respond-with-body.js

+self.addEventListener('fetch', e => {
+    if (new URL(e.request.url).search != '?respond-with-body') {
+        return;
+    }
+    e.respondWith(new Promise(r => {
+        e.request.text().then(t => {
+            var data = {
+                'credentials': e.request.credentials,
+                'body': t
+            };
+            r(new Response(JSON.stringify(data)));
+        })
+    }));
+});
+