bindings: Fixes layouttests when moving attributes to prototype chains.

LayoutTests/http/tests/resources/testharness.js

Index: LayoutTests/http/tests/resources/testharness.js
diff --git a/LayoutTests/http/tests/resources/testharness.js b/LayoutTests/http/tests/resources/testharness.js
index 2b3eaaa3ec7ac1b7f83c21cdb170515f0c80ef7c..732e9a167ce844928e2edf8a17d27e58cc53959d 100644
--- a/LayoutTests/http/tests/resources/testharness.js
+++ b/LayoutTests/http/tests/resources/testharness.js
@@ -390,6 +390,13 @@ policies and contribution forms [3].
  *   assert that an object that is an instance of some interface has the
  *   attribute attribute_name following the conditions specified by WebIDL
  *
+ * assert_will_be_idl_attribute(object, attribute_name, description)
+ *   assert that an 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.
+ *
  * assert_readonly(object, property_name, description)
  *   assert that property property_name on object is readonly
  *
@@ -1351,6 +1358,28 @@ policies and contribution forms [3].
     expose(_assert_inherits("assert_inherits"), "assert_inherits");
     expose(_assert_inherits("assert_idl_attribute"), "assert_idl_attribute");
 
+    // FIXME: Remove assert_will_be_idl_attribute once we finish the transition
+    // of moving the DOM attributes to prototype chains.
+    function assert_will_be_idl_attribute(object, property_name, description) {
+        assert(typeof object === "object",
+               "assert_will_be_idl_attribute", description,
+               "provided value is not an object");
+
+        assert("hasOwnProperty" in object,
+               "assert_will_be_idl_attribute", description,
+               "provided value is an object but has no hasOwnProperty method");
+
+        // Do not test if property_name is not an own property because
+        // property_name is in the middle of the transition to a prototype
+        // chain.
+
+        assert(property_name in object,
+               "assert_will_be_idl_attribute", description,
+               "property ${p} not found in the provided value",
+               {p:property_name});
+    }
+    expose(assert_will_be_idl_attribute, "assert_will_be_idl_attribute");
+
     function assert_readonly(object, property_name, description)
     {
          var initial_value = object[property_name];