IDL compiler: [URL] for setters

Test-only change (now that [Reflect] implemented),
as [URL] only affects getters.
(Could've landed with [Reflect] CL, admittedly.)

BUG=239771
NOTRY=true

Review URL: https://codereview.chromium.org/34353002

git-svn-id: svn://svn.chromium.org/blink/trunk@160138 bbb929c8-8fbe-4397-9dbb-9b2b20218538