IDL compiler: [KeepAttributeAliveForGC] for setters

Test-only change, as this only affects getters, and it's already implemented.
(Meaningful test though, as this extended attribute is a nop for read only
attributes, so this is checking behavior for writable attributes.)

Docs:
https://sites.google.com/a/chromium.org/dev/blink/webidl/blink-idl-extended-attributes#TOC-KeepAttributeAliveForGC-a-

BUG=239771
NOTRY=true

Review URL: https://codereview.chromium.org/31843002

git-svn-id: svn://svn.chromium.org/blink/trunk@160065 bbb929c8-8fbe-4397-9dbb-9b2b20218538