bindings: Fixes layouttests when moving attributes to prototype chains.

LayoutTests/http/tests/resources/testharness-helpers.js

 /*
  * testharness-helpers contains various useful extensions to testharness.js to
  * allow them to be used across multiple tests before they have been
  * upstreamed. This file is intended to be usable from both document and worker
  * environments, so code should for example not rely on the DOM.
  */
 
 // Returns a promise that fulfills after the provided |promise| is fulfilled.
 // The |test| succeeds only if |promise| rejects with an exception matching
 // |code|. Accepted values for |code| follow those accepted for assert_throws().
@@ skipping 104 matching lines @@
 // elements as determined by assert_object_equals(). The corresponding elements
 // must occupy corresponding indices in their respective arrays.
 function assert_array_objects_equals(actual, expected, description) {
   assert_true(Array.isArray(actual), description);
   assert_equals(actual.length, expected.length, description);
   actual.forEach(function(value, index) {
       assert_object_equals(value, expected[index],
                            description + ' : object[' + index + ']');
     });
 }
+
+// Asserts that |object| that is an instance of some interface has the attribute
+// |attribute_name| following the conditions specified by WebIDL, but it's
+// acceptable that the attribute |attribute_name| is an own property of the
+// object because we're in the middle of moving the attribute to a prototype
+// chain.  Once we complete the transition to prototype chains,
+// assert_will_be_idl_attribute must be replaced with assert_idl_attribute
+// defined in testharness.js.
+//
+// FIXME: Remove assert_will_be_idl_attribute once we complete the transition
+// of moving the DOM attributes to prototype chains.  (http://crbug.com/43394)
+function assert_will_be_idl_attribute(object, attribute_name, description) {
+  assert_true(typeof object === "object", description);
+
+  assert_true("hasOwnProperty" in object, description);
+
+  // Do not test if |attribute_name| is not an own property because
+  // |attribute_name| is in the middle of the transition to a prototype
+  // chain.  (http://crbug.com/43394)
+
+  assert_true(attribute_name in object, description);
+}