Allow HandleOrOffThreadHandle to implicitly convert to any of
Handle, MaybeHandle, and OffThreadHandle, so that it can be used
in Isolate-templated code with fewer 'get' calls.

As a drive-by, allow OffThreadHandle to have an OffThreadIsolate
parameter (so that HandleFor<Isolate, Foo>(x, isolate) works), and
use a static_cast rather than a T::cast in the implicit upcast
constructor of OffThreadHandle to prevent implicit down-casts.

Bug: chromium:1011762
Change-Id: I226f3a48b0f3b5e0347e6325be1cbd482c9048ed
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2042100
Auto-Submit: Leszek Swirski <leszeks@chromium.org>
Commit-Queue: Igor Sheludko <ishell@chromium.org>
Reviewed-by: Igor Sheludko <ishell@chromium.org>
Cr-Commit-Position: refs/heads/master@{#66157}