Reuse the last LAB's unused area for further newspace allocation.

This is relevant when we expect GCs that use evacuation to compact down
new space to just live bytes for single tasks.

BUG=chromium:651354

Change-Id: Ic418521d98f418a93d3748b824e3ddb6ff7a40c3
Reviewed-on: https://chromium-review.googlesource.com/451398
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Reviewed-by: Hannes Payer <hpayer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#43670}