Previously Isolate and Factory relied on the undefined behavior of
reinterpret_cast to switch between the two unrelated classes (which worked
because Factory had no data members).

With Isolate inheriting from Factory, it's now possible to switch between the
two classes using c-style casts. These are allowed under the C++ standard.

The inheritance is private which allows the continuing separation of the
Factory and Isolate namespaces.

This is a defensive clean-up, since ubsan does not yet detect the previous
undefined behavior.

Bug: v8:3770
Change-Id: I0ccf09f1d34f747550812ce698ab7e182812409e
Reviewed-on: https://chromium-review.googlesource.com/1010122Reviewed-by: Jakob Kummerow <jkummerow@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Commit-Queue: Dan Elphick <delphick@chromium.org>
Cr-Commit-Position: refs/heads/master@{#52615}