IDL: Support optional argument default value syntax

Adds support for parsing default values of different types, but
only handles null default values when generating code.

Replaces existing

  [Default=Null] optional SomeInterface arg
  [Default=NullString] optional DOMString arg

with the now equivalent

  optional SomeInterface arg = null
  optional DOMString arg = null

in IDL files, and drops support for those [Default] attributes.

No changes to generated code.

BUG=258153

Review URL: https://codereview.chromium.org/312683005

git-svn-id: svn://svn.chromium.org/blink/trunk@176200 bbb929c8-8fbe-4397-9dbb-9b2b20218538