The only valid way to define a GCed type T is by inheriting from
GarbageCollected<T>. Since this is prone to typos (see tests), add a
simple check that covers most interesting use cases.

The static assert covers
  A -> B -> GarbageCollected<C>

The static assert does not cover
 A -> B -> C -> GarbageCollected<B>

(In order to do so, we would need __direct_bases() support which is
not yet available for C++.)

Bug: pdfium:1670, chromium:1056170
Change-Id: I494de48992f8ba9a1f0f9daad60584d828717403
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2810415Reviewed-by: Omer Katz <omerkatz@chromium.org>
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#73854}