IDL compiler: Variadic arguments for built-in types

Variadic arguments of interface type require type checking,
so I'll do that in a separate CL.

BUG=239771
NOTRY=true

Review URL: https://codereview.chromium.org/50403002

git-svn-id: svn://svn.chromium.org/blink/trunk@160823 bbb929c8-8fbe-4397-9dbb-9b2b20218538