Expose mojo bindings to subframes in layout tests.

third_party/WebKit/LayoutTests/harness-tests/mojo-helpers.html

Index: third_party/WebKit/LayoutTests/harness-tests/mojo-helpers.html
diff --git a/third_party/WebKit/LayoutTests/harness-tests/mojo-helpers.html b/third_party/WebKit/LayoutTests/harness-tests/mojo-helpers.html
index 005b8cdc87105a54c20f936509450762abc785ef..b68dda0a71ec111645c27da2f28d69e3d28d346c 100644
--- a/third_party/WebKit/LayoutTests/harness-tests/mojo-helpers.html
+++ b/third_party/WebKit/LayoutTests/harness-tests/mojo-helpers.html
@@ -73,4 +73,19 @@ mojo_test(mojo => {
       assert_array_equals(response.handles, []);
     });
 }, 'Test code can receive response messages from mock services.');
+
+mojo_test(() => {
+  return new Promise(resolve => {
+    let iframe = document.createElement('iframe');
+    let listener = result => {
+      document.body.removeChild(iframe);
+      window.removeEventListener('message', listener);
+      assert_true(result.data);
+      resolve();
+    }
+    window.addEventListener('message', listener);
+    iframe.setAttribute('src', 'resources/mojo-helpers-inner.html');
+    document.body.appendChild(iframe);
+  });
+}, 'Mojo bindings are accessible from iframes');
 </script>