1) JSWeakRef and WeakCell targets are HeapObjects (SMIs are no longer used for
signalling cleared / dead JSWeakRefs / WeakCells.) Make this explicit.

2) There's no need to assert that the target of JSWeakRef cannot be undefined
when handled by MarkCompactCollector::ClearJSWeakRefs, since the code handles
undefined just fine. (The removed comment was true though, since this is the only place
which can set the target to undefined. But maybe in the future there's some
other way to clear a JSWeakRef, e.g., explicit API for it.)

BUG=v8:8179

Change-Id: I762c2b4487770712c7538be799dc188943c92587
Reviewed-on: https://chromium-review.googlesource.com/c/1445986Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Reviewed-by: Sathya Gunasekaran <gsathya@chromium.org>
Commit-Queue: Marja Hölttä <marja@chromium.org>
Cr-Commit-Position: refs/heads/master@{#59237}