IDL compiler: EventHandler for setters

Ugly special case, handled as generically as possible.
Also some cleanup of previous EventHandler includes special casing.

BUG=239771
NOTRY=true

Review URL: https://codereview.chromium.org/30493002

git-svn-id: svn://svn.chromium.org/blink/trunk@160051 bbb929c8-8fbe-4397-9dbb-9b2b20218538