service worker: Fix navigation-preload/resource-timing.https.html test

Before this CL, this test tried to get performance entries right after
Response#text() is resolved, but Chrome doesn't guarantee that there
will be a performance entry for the response at that point. This CL
changes the test to use PerformanceObserver so that we can make sure
that there will be a performance entry for the response.

Bug: 872078
Change-Id: I6ecfcb8e0721df541d5a32f56ec4c062efbffdae
Reviewed-on: https://chromium-review.googlesource.com/1170679
Commit-Queue: Kenichi Ishibashi <bashi@chromium.org>
Reviewed-by: Kunihiko Sakamoto <ksakamoto@chromium.org>
Reviewed-by: Matt Falkenhagen <falken@chromium.org>
Cr-Commit-Position: refs/heads/master@{#583483}

third_party/WebKit/LayoutTests/external/wpt/service-workers/service-worker/navigation-preload/resources/resource-timing-worker.js

+async function wait_for_performance_entries(url) {
+  let entries = performance.getEntriesByName(url);
+  if (entries.length > 0) {
+    return entries;
+  }
+  return new Promise((resolve) => {
+    new PerformanceObserver((list) => {
+      const entries = list.getEntriesByName(url);
+      if (entries.length > 0) {
+        resolve(entries);
+      }
+    }).observe({ entryTypes: ['resource'] });
+  });
+}
+
 self.addEventListener('activate', event => {
     event.waitUntil(self.registration.navigationPreload.enable());
   });
 
 self.addEventListener('fetch', event => {
+    let headers;
     event.respondWith(
       event.preloadResponse
           .then(response => {
-            var headers = response.headers;
-            return response.text().then(text =>
-              new Response(
-                JSON.stringify({
-                  decodedBodySize: headers.get('X-Decoded-Body-Size'),
-                  encodedBodySize: headers.get('X-Encoded-Body-Size'),
-                  timingEntries: performance.getEntriesByName(event.request.url)
-                }),
-                {headers: {'Content-Type': 'text/html'}}));
-          }));
+            headers = response.headers;
+            return response.text()
+          })
+          .then(_ => wait_for_performance_entries(event.request.url))
+          .then(entries =>
+            new Response(
+              JSON.stringify({
+                decodedBodySize: headers.get('X-Decoded-Body-Size'),
+                encodedBodySize: headers.get('X-Encoded-Body-Size'),
+                timingEntries: entries
+              }),
+              {headers: {'Content-Type': 'text/html'}})));
   });