Update trailing comma comment per current spec

Trailing commas were accepted in Enums, but rejected in Extended Attributes
for Web IDL spec (marked WONTFIX).
Updated comment to reflect this, removing FIXME.
W3C 22156: Allow trailing commas in Web IDL lists
https://www.w3.org/Bugs/Public/show_bug.cgi?id=22156

Review URL: https://chromiumcodereview.appspot.com/21904003

git-svn-id: svn://svn.chromium.org/blink/trunk@155519 bbb929c8-8fbe-4397-9dbb-9b2b20218538