Introduce support for ExtendedAttributeIdentList in the IDL parser.

BUG=399653

Review URL: https://codereview.chromium.org/447543002

git-svn-id: http://src.chromium.org/chrome/trunk/src/tools/idl_parser@287775 4ff67af0-8c30-449e-8e8b-ad334ec8d88c