IDL compiler: [RuntimeEnabled] interfaces

Straightforward, also fixes an indentation error in the Perl.
(Slightly awkward to do cleanly, though Jinja isn't bad.)

BUG=239771

Review URL: https://codereview.chromium.org/103233005

git-svn-id: svn://svn.chromium.org/blink/trunk@163159 bbb929c8-8fbe-4397-9dbb-9b2b20218538