Perl IDL parser: allow extended attributes to have more than 2 values

The (Perl) IDL parser has a bug, limiting extended attribute value lists
to being length at most 2.
This hasn't been a problem in the past, since this isn't used, but caused
a problem in a recent CL where arbitrarily long lists are common.
This fixes it; it's a 2-line fix.

Relevant CL:
Support IDL attributes limited to only known values.
https://codereview.chromium.org/124763002/

Review URL: https://codereview.chromium.org/124943002

git-svn-id: svn://svn.chromium.org/blink/trunk@164500 bbb929c8-8fbe-4397-9dbb-9b2b20218538