service worker: Upstream sandboxed iframe test to WPT.

This test is a fine subject matter for WPT.

Bug: 486308, 688116
Change-Id: I91d8484286094c695fe2219f0c017e25e8bfd34c
Reviewed-on: https://chromium-review.googlesource.com/759168Reviewed-by: Tsuyoshi Horo <horo@chromium.org>
Commit-Queue: Matt Falkenhagen <falken@chromium.org>
Cr-Commit-Position: refs/heads/master@{#515030}

third_party/WebKit/LayoutTests/external/wpt/service-workers/service-worker/resources/test-helpers.sub.js

@@ -255,3 +255,13 @@ function register_using_link(script, options) {
       })
     .then(() => navigator.serviceWorker.getRegistration(scope));
 }
+
+function with_sandboxed_iframe(url, sandbox) {
+  return new Promise(function(resolve) {
+      var frame = document.createElement('iframe');
+      frame.sandbox = sandbox;
+      frame.src = url;
+      frame.onload = function() { resolve(frame); };
+      document.body.appendChild(frame);
+    });
+}