This lets us remove the unsafe object<T>() getter.

Bug: v8:7790
Change-Id: Ie438c68d4c96f1525eee5afd252523b222dc8f53
Reviewed-on: https://chromium-review.googlesource.com/c/1288411Reviewed-by: Jaroslav Sevcik <jarin@chromium.org>
Reviewed-by: Maya Lekova <mslekova@chromium.org>
Commit-Queue: Georg Neis <neis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#56761}