Left-trimming only works when there is a single reference to the
backing store from the JS object. Main thread handles are an exception
to this rule because it is not feasible to ensure that no such
leftover handles may store such stale pointers.

FixStaleLeftTrimmedHandlesVisitor clears such references
in main thread handles, such that the GC never tries to visit them. This
CL renames this class to ClearStaleLeftTrimmedHandlesVisitor to
emphasize that such slots are cleared rather than "fixed up" to point
to the new object start.

Previously ClearStaleLeftTrimmedHandlesVisitor was used for local
and persistent handles as well. Starting with this CL, stale references
to left-trimmed objects are only allowed in main thread handles.
https://crrev.com/c/2928502 enabled us to be more restrictive here.

Change-Id: If4db0630f1df2d6c3fe5f242bf866c57a8ae2969
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2944807Reviewed-by: Georg Neis <neis@chromium.org>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Commit-Queue: Dominik Inführ <dinfuehr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#74989}