Add "platforms" key in IDL schema compiler.

tools/json_schema_compiler/model.py

 # Copyright (c) 2012 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.
 
 import os.path
 
 from json_parse import OrderedDict
 from memoize import memoize
 
 class ParseException(Exception):
@@ skipping 468 matching lines @@
 class Platforms(object):
   """Enum of the possible platforms.
   """
   CHROMEOS = _PlatformInfo("chromeos")
   CHROMEOS_TOUCH = _PlatformInfo("chromeos_touch")
   LINUX = _PlatformInfo("linux")
   MAC = _PlatformInfo("mac")
   WIN = _PlatformInfo("win")
 
 def _GetPlatforms(json):
-  if 'platforms' not in json:
+  if 'platforms' not in json or json['platforms'] == None:
     return None
+  # Sanity check: platforms should not be an empty list.
+  if not json['platforms']:
+    raise ValueError('"platforms" cannot be an empty list')
   platforms = []
   for platform_name in json['platforms']:
     for platform_enum in _Enum.GetAll(Platforms):
       if platform_name == platform_enum.name:
         platforms.append(platform_enum)
         break
   return platforms