This change relands the last part of https://crrev.com/c/2601880 .

ScopeInfo has a vestigial 'length' field from when it used to be a
FixedArray. This change removes that field, which saves some memory.

More specifically:

- Make ScopeInfo inherit from HeapObject, not FixedArrayBase which
  supplied the 'length' field.
- Change FactoryBase::NewScopeInfo to allocate the updated object shape.
  It maintains the existing behavior of filling the newly-allocated
  object with undefined, even though that's not a valid ScopeInfo and
  further initialization is required.
- Change a few length computations to use HeapObject::kHeaderSize rather
  than FixedArray::kHeaderSize.
- Remove an unnecessary heap verifier function.

Change-Id: I9b3980157568fdb0402fa31660949966b401fd31
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2733037Reviewed-by: Dominik Inführ <dinfuehr@chromium.org>
Commit-Queue: Seth Brenith <seth.brenith@microsoft.com>
Cr-Commit-Position: refs/heads/master@{#73278}