JSArrays and JSArrayBuffers are very different animals. As such,
split the js-array.h header into two parts.

R=ulan@chromium.org,mstarzinger@chromium.org

Bug: v8:5402
Cq-Include-Trybots: luci.chromium.try:linux_chromium_rel_ng
Change-Id: I82f987ecea3e2e1ceaf8f8962a2b88165558c57e
Reviewed-on: https://chromium-review.googlesource.com/1177760Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
Commit-Queue: Ben Titzer <titzer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#55183}