Add layout tests for Service Worker fallbacked subresource requests.

LayoutTests/http/tests/serviceworker/resources/fetch-request-fallback-iframe.html

Index: LayoutTests/http/tests/serviceworker/resources/fetch-request-fallback-iframe.html
diff --git a/LayoutTests/http/tests/serviceworker/resources/fetch-request-fallback-iframe.html b/LayoutTests/http/tests/serviceworker/resources/fetch-request-fallback-iframe.html
index f00d24e37a8270cf65c0322c2dabb5d1a20910d2..37c7c62ffaf38b0a906da928a56c054e04d9a873 100644
--- a/LayoutTests/http/tests/serviceworker/resources/fetch-request-fallback-iframe.html
+++ b/LayoutTests/http/tests/serviceworker/resources/fetch-request-fallback-iframe.html
@@ -12,4 +12,21 @@ function xhr(url) {
       request.send();
     });
 }
+
+function load_image(url, cross_origin) {
+  return new Promise(function(resolve, reject) {
+      var img = document.createElement('img');
+      document.body.appendChild(img);
+      img.onload = function() {
+        resolve();
+      };
+      img.onerror = function() {
+        reject();
+      };
+      if (cross_origin != '') {
+        img.crossOrigin = cross_origin;
+      }
+      img.src = url;
+    });
+}
 </script>