message: dawn_json_generator: Assert that enum values are unique.
  This will be useful when we add all texture formats because there are
  more than 50 of them and we want to make sure no two enum values
  collide.
diff: @@ -67,6 +67,13 @@ class EnumType(Type):
  Type.__init__(self, name, json_data)
  self.values = [EnumValue(Name(m['name']), m['value']) for m in self.json_data['values']]
  + # Assert that all values are unique in enums
  + all_values = set()
  + for value in self.values:
  + if value.value in all_values:
  + raise Exception("Duplicate value {} in enum {}".format(value.value, name))
  + all_values.add(value.value)
  +
  BitmaskValue = namedtuple('BitmaskValue', ['name', 'value'])
  class BitmaskType(Type):
  def __init__(self, name, json_data):
  