Add an implementation path option to json_schema_compiler

ui/accessibility/accessibility.gyp

 # Copyright 2013 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.
 
 {
   'variables': {
     'chromium_code': 1,
   },
 
   'targets': [
@@ skipping 72 matching lines @@
       'variables': {
         'chromium_code': 1,
         'schema_files': [
           'ax_enums.idl',
         ],
         'non_compiled_schema_files': [],
         'cc_dir': 'ui/accessibility',
         # TODO(dtseng): Change this once all files under ui/accessibility
         # namespaced under ui::ax.
         'root_namespace': '',
+        'impl_dir': ''
       },
     },
   ],
 }