This CL is a pure code move based on top of
https://chromium-review.googlesource.com/c/v8/v8/+/899365

Having it all in the header was becoming more and more tedious:
requiring large rebuilds for impl changes and
requiring exporting unrelated symbols merely so that unittests
could link
@ https://chromium-review.googlesource.com/c/v8/v8/+/899365
and https://chromium-review.googlesource.com/c/v8/v8/+/904523/2

R=mlippautz@chromium.org

Bug: chromium:651354
Change-Id: Ib34043d061dd3b1221cd06799eddc888090fe1c1
Reviewed-on: https://chromium-review.googlesource.com/904167Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Commit-Queue: Gabriel Charette <gab@chromium.org>
Cr-Commit-Position: refs/heads/master@{#51132}