Exposing it inside Internals was a hack. The downside of this CL is that heap
object tagging is in two places now (v8.h and globals.h).

BUG=v8:7308

Cq-Include-Trybots: luci.chromium.try:linux_chromium_rel_ng
Change-Id: Ic7115ab20d67109dd2b62c772d52eeb84fa7d9f7
Reviewed-on: https://chromium-review.googlesource.com/968423Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Reviewed-by: Yang Guo <yangguo@chromium.org>
Reviewed-by: Hannes Payer <hpayer@chromium.org>
Commit-Queue: Marja Hölttä <marja@chromium.org>
Cr-Commit-Position: refs/heads/master@{#52093}