- Add a a "d8" global variable where d8 can provide helpers.
  This in in preparation of adding d8.log for testing our log parsers
  written in JavaScript.

- Separate d8 helper creation into individual functions.

Bug: v8:1064
Change-Id: I84e434452463afb93ae403f890d8841b20b00703
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2400990Reviewed-by: Toon Verwaest <verwaest@chromium.org>
Commit-Queue: Camillo Bruni <cbruni@chromium.org>
Cr-Commit-Position: refs/heads/master@{#69801}