Allow cleared references to be created with an OffThreadIsolate.
This includes allowing isolate_root to be accessed from the
OffThreadIsolate, for pointer decompression.

Bug: chromium:1075999
Change-Id: I62e0fe2c1c6166a7b816593ae1ec5ddb1c25d861
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2183911
Commit-Queue: Leszek Swirski <leszeks@chromium.org>
Reviewed-by: Igor Sheludko <ishell@chromium.org>
Auto-Submit: Leszek Swirski <leszeks@chromium.org>
Cr-Commit-Position: refs/heads/master@{#67599}