There is no need to remove an object pointer's heap object
tag when all we want is the Page that it's on. Also, apply
to IncrementalMarking's writebarrier the optimization that
crrev.com/e570e673 brought
to the old-to-new barrier.

Change-Id: Ic9328d7d6f5c01073288a3e87931ea6095750740
Reviewed-on: https://chromium-review.googlesource.com/1029413
Commit-Queue: Jakob Kummerow <jkummerow@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Reviewed-by: Hannes Payer <hpayer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#52824}