Always specify enumerator argument of SetIndexedPropertyHandler.

Specify 0 for enumerator argument of SetIndexedPropertyHandler
even if no enumerator so that python code generator can
easily generate the code with template.

We already call with 0 for SetNamedPropertyHandler.

R=haraken@chromium.org
BUG=239771
TEST=TestCustomAccessors.idl

Review URL: https://chromiumcodereview.appspot.com/19520003

git-svn-id: svn://svn.chromium.org/blink/trunk@154388 bbb929c8-8fbe-4397-9dbb-9b2b20218538