IDL compiler: Simplify [Custom], [CustomSetter] code

Per:
https://codereview.chromium.org/30533002/#msg8

BUG=239771
NOTRY=true

Review URL: https://codereview.chromium.org/31143003

git-svn-id: svn://svn.chromium.org/blink/trunk@160047 bbb929c8-8fbe-4397-9dbb-9b2b20218538