[unified-heap] Use up-to-date V8 api

Bug: 843903
Change-Id: I9271138dfcdf72cd67965ada3b3e8b03b3b89222
Reviewed-on: https://chromium-review.googlesource.com/c/1277803
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#599613}
6 files changed