IDL compiler: Variadic wrapper type arguments

Bulky special case; this is used one place:
Document.idl:
[RuntimeEnabled=Touch] TouchList createTouchList(Touch... touches);

BUG=239771
NOTRY=true

Review URL: https://codereview.chromium.org/50503003

git-svn-id: svn://svn.chromium.org/blink/trunk@160827 bbb929c8-8fbe-4397-9dbb-9b2b20218538