IDL: Avoid double type-check for variadic interface type arguments

With [TypeChecking=Interface], two type-checks would be emitted for
variadic arguments; one as if the argument wasn't variadic, checking only
the first actual argument, and then another one checking each actual
argument.

Review URL: https://codereview.chromium.org/359763002

git-svn-id: svn://svn.chromium.org/blink/trunk@177094 bbb929c8-8fbe-4397-9dbb-9b2b20218538