Merging the temporary HeapObjectPtr back into HeapObject.

Bug: v8:3770
Change-Id: I5bcd23ca2f5ba862cf5b52955dca143e531c637b
Reviewed-on: https://chromium-review.googlesource.com/c/1386492
Commit-Queue: Jakob Kummerow <jkummerow@chromium.org>
Reviewed-by: Clemens Hammacher <clemensh@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Reviewed-by: Michael Stanton <mvstanton@chromium.org>
Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Cr-Commit-Position: refs/heads/master@{#58410}