Support WebIDL's ExtendedAttributeIdentList in the Blink parser

A change was made to the WebIDL parser on July 29 to support a list of
identifiers as part of an extended attribute's value. This cleanly maps
to the concept of the IdentAndList (a Blink extension) we already had.

All uses of the IdentAndList have been updated to use the specification-
compliant syntax. The IdentOrList remains in place.

BUG=399653

Review URL: https://codereview.chromium.org/434133002

git-svn-id: svn://svn.chromium.org/blink/trunk@179736 bbb929c8-8fbe-4397-9dbb-9b2b20218538