IDL compiler: enum types for setters

The Perl code looks *strangely* familiar (^.-).

BUG=239771
NOTRY=true

Review URL: https://codereview.chromium.org/29123006

git-svn-id: svn://svn.chromium.org/blink/trunk@160007 bbb929c8-8fbe-4397-9dbb-9b2b20218538