Add a helper for implementing Promise-valued properties.

Source/bindings/v8/v8.gypi

 # Copyright 2014 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 {
     'includes': [
         # FIXME: should move bindings_v8_files to bindings/core/v8 or
         # bindings/modules/v8.
         '../core/v8/custom/custom.gypi',
         '../modules/v8/generated.gypi',
@@ skipping 60 matching lines @@
             'ScriptHeapSnapshot.cpp',
             'ScriptHeapSnapshot.h',
             'ScriptPreprocessor.cpp',
             'ScriptPreprocessor.h',
             'ScriptProfiler.cpp',
             'ScriptProfiler.h',
             'ScriptPromise.cpp',
             'ScriptPromise.h',
             'ScriptPromiseResolver.cpp',
             'ScriptPromiseResolver.h',
+            'ScriptPromiseProperty.h',
+            'ScriptPromisePropertyBase.cpp',
+            'ScriptPromisePropertyBase.h',
             'ScriptRegexp.cpp',
             'ScriptRegexp.h',
             'ScriptSourceCode.h',
             'ScriptState.cpp',
             'ScriptState.h',
             'ScriptString.cpp',
             'ScriptString.h',
             'ScriptValue.cpp',
             'ScriptValue.h',
             'ScriptWrappable.h',
@@ skipping 62 matching lines @@
             'WorkerScriptDebugServer.cpp',
             'WorkerScriptDebugServer.h',
             'WrapperTypeInfo.h',
             'npruntime.cpp',
             'npruntime_impl.h',
             'npruntime_priv.h',
         ],
         'bindings_v8_unittest_files': [
             'IDBBindingUtilitiesTest.cpp',
             'ScriptPromiseTest.cpp',
+            'ScriptPromisePropertyTest.cpp',
         ],
     },
 }