Split bindings/v8/custom into core and modules.

Source/bindings/modules/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': [
-        'v8/v8.gypi',
+        'custom/custom.gypi',
     ],
     'variables': {
-        'bindings_dir': '.',
-        'bindings_unittest_files': [
-            '<@(bindings_v8_unittest_files)',
+        'bindings_modules_v8_files': [
+            '<@(bindings_modules_v8_custom_files)',
         ],
     },
 }