Add LayoutTest for ServiceWorker's sandbox iframe handling.

LayoutTests/http/tests/serviceworker/resources/test-helpers.js

Index: LayoutTests/http/tests/serviceworker/resources/test-helpers.js
diff --git a/LayoutTests/http/tests/serviceworker/resources/test-helpers.js b/LayoutTests/http/tests/serviceworker/resources/test-helpers.js
index 1748c4994f041a5a6aecf75788f0ccb0a812ad1d..2a8e7122565b9f9b7ed39d424b7d3f284bc56c5f 100644
--- a/LayoutTests/http/tests/serviceworker/resources/test-helpers.js
+++ b/LayoutTests/http/tests/serviceworker/resources/test-helpers.js
@@ -55,6 +55,16 @@ function with_iframe(url) {
     });
 }
 
+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);
+    });
+}
+
 function normalizeURL(url) {
   return new URL(url, self.location).toString().replace(/#.*$/, '');
 }