Remove OffThreadHandle, HandleOrOffThreadHandle, and HandleFor, and
make the OffThreadIsolate allocate "real" Handles. Rather than using
the main-thread Isolate's handle scopes, these off-thread Handles are
backed by a Zone, which is tied to the lifetime of the nearest
OffThreadHandleScope. Eventually, we'll likely want to merge the
implementation of OffThreadHandleScope and HandleScope, but currently
the latter is too tightly coupled to the main thread to do so.

Bug: chromium:1011762
Change-Id: I2a6361931fe3f90a7bef4cc28ee42155fa8d062f
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2071865Reviewed-by: Ross McIlroy <rmcilroy@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Reviewed-by: Toon Verwaest <verwaest@chromium.org>
Commit-Queue: Leszek Swirski <leszeks@chromium.org>
Cr-Commit-Position: refs/heads/master@{#66516}