Some preperatory refactoring to allow observation of inline allocations
from Old Space.

BUG=chromium:633920

Change-Id: Ia1232591860729fcd8942d816aa454171d3aec33
Reviewed-on: https://chromium-review.googlesource.com/617923Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Commit-Queue: Ali Ijaz Sheikh <ofrobots@google.com>
Cr-Commit-Position: refs/heads/master@{#47413}