Explicit nullification aims to simplify migration to Oilpan, in the
case when unique_ptrs are converted to Member and user code relies on
source pointers to be in "empty" state.

Change-Id: Ia54137d53ca03f93932b3c1f2eaba439a416a06e
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2379857Reviewed-by: Omer Katz <omerkatz@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Auto-Submit: Anton Bikineev <bikineev@chromium.org>
Commit-Queue: Anton Bikineev <bikineev@chromium.org>
Cr-Commit-Position: refs/heads/master@{#69603}