IDL compiler: [Conditional] for setters

Simple change, with some whitespace changes on Perl for consistency
and simplicity.

BUG=239771

Review URL: https://codereview.chromium.org/28333003

git-svn-id: svn://svn.chromium.org/blink/trunk@160004 bbb929c8-8fbe-4397-9dbb-9b2b20218538