Fix _first_enum_value usage on make_css_property_names.py

Source/build/scripts/css_properties.py

 #!/usr/bin/env python
 # 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.
 
 import in_generator
 from name_utilities import lower_first
 
 
 class CSSProperties(in_generator.Writer):
@@ skipping 34 matching lines @@
     }
 
     def __init__(self, file_paths):
         in_generator.Writer.__init__(self, file_paths)
 
         properties = self.in_file.name_dictionaries
 
         self._aliases = [property for property in properties if property['alias_for']]
         properties = [property for property in properties if not property['alias_for']]
 
-        # StylePropertyMetadata additionally assumes there are under 1024 properties.
-        assert len(properties) < 512, 'Property aliasing expects there are under 512 properties.'
-
         # We currently assign 0 to CSSPropertyInvalid
         self._first_enum_value = 1
+
+        # StylePropertyMetadata additionally assumes there are under 1024 properties.
+        assert self._first_enum_value + len(properties) < 512, 'Property aliasing expects there are under 512 properties.'
+
         for offset, property in enumerate(properties):
             property['property_id'] = css_name_to_enum(property['name'])
             property['upper_camel_name'] = camelcase_css_name(property['name'])
             property['lower_camel_name'] = lower_first(property['upper_camel_name'])
             property['enum_value'] = self._first_enum_value + offset
             property['is_internal'] = property['name'].startswith('-internal-')
 
         self._properties_including_aliases = properties
         self._properties = {property['property_id']: property for property in properties}
 
@@ skipping 14 matching lines @@
     """
     return ''.join(word.capitalize() for word in css_name.split('-'))
 
 
 def css_name_to_enum(css_name):
     return 'CSSProperty' + camelcase_css_name(css_name)
 
 
 def css_alias_name_to_enum(css_name):
     return 'CSSPropertyAlias' + camelcase_css_name(css_name)