[vm] Handle WSRs more generally in v8 snapshot profile writing.

Instead of special-casing the current fields that may have
WeakSerializationReferences, handle WSRs appearing as elements
or properties of objects more generally. This removes existing
special casing and avoids the need for it in case of new
future uses of WSRs.

For artificial nodes being added for dropped
WeakSerializationReference targets, add them as kArtificial nodes
(not kSnapshot) that has the original offset (element) or
name (property). The replacement is added as a kSnapshot node
that has a negative offset with the same magnitude as the
artificial node (element) or ":real_<property name>" (property).
This simplifies the work done in pkg/vm_snapshot_analysis to
use the artificial nodes instead of replacement ones for
reassembling hierarchies and the like.

This CL also cleans up the old SerializerWritingObjectScope
class, both moving it to Serializer::WritingObjectScope and
allowing nesting of WritingObjectScopes with the correct
semantics.

Thanks to this, not only can we recur when under a WritingObjectScope
instead of lifting recursion outside of those scopes, but we can
also create artificial nodes for non-empty per-code object pools and
static call target tables instead of attributing their contents
as supposed elements of the Code object being written.

TEST=pkg/vm_snapshot_analysis/test/instruction_sizes_test

Cq-Include-Trybots: luci.dart.try:pkg-linux-release-try,pkg-mac-release-try,pkg-win-release-try
Change-Id: Ib945c5afcd89b1458b8be3559b6eae24048aba2f
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/194243
Commit-Queue: Tess Strickland <sstrickl@google.com>
Reviewed-by: Ryan Macnak <rmacnak@google.com>
Reviewed-by: Vyacheslav Egorov <vegorov@google.com>