and prepare for adding an EmbedderDataSlot which will occupy two tagged slots.

Bug: v8:8477, v8:8238, v8:7703
Change-Id: I8aa0a0c9b64835ad7f847033bcc55dd3b3c43563
Reviewed-on: https://chromium-review.googlesource.com/c/1344153
Commit-Queue: Igor Sheludko <ishell@chromium.org>
Reviewed-by: Toon Verwaest <verwaest@chromium.org>
Cr-Commit-Position: refs/heads/master@{#57669}