Following up on adding n-ary nodes, this extends the parser to support
n-ary comma operations, including support for n-ary arrow function
parameters.

Bug: v8:6964
Bug: chromium:777302
Change-Id: Iba9c93b9eaa5a0870815b4fa29e84aa9d0c511e2
Reviewed-on: https://chromium-review.googlesource.com/735156
Commit-Queue: Leszek Swirski <leszeks@chromium.org>
Reviewed-by: Marja Hölttä <marja@chromium.org>
Reviewed-by: Ross McIlroy <rmcilroy@chromium.org>
Cr-Commit-Position: refs/heads/master@{#48927}