* GetThreadSlot was missing const (Blink CL failed to build).
* Aligning IsEmptyThreadSafe implementation with IsEmpty.

Bug: chromium:1108537
Change-Id: I9c07e9a0e7f029cf3d70b4d2507900b600d064cc
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2330027
Auto-Submit: Omer Katz <omerkatz@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Commit-Queue: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#69157}