In preparation for sharing RO_SPACE between all Isolates within a
process, this first pulls RO_SPACE out of the Startup snapshot and puts
it in its own ReadOnly snapshot.

The snapshot is first populated with the read-only roots. After that the
StartupSerializer serializes as before but starting from the first
mutable root. References to objects in the ReadOnly snapshot that aren't
themselves roots are added to a new cache called ReadOnlyObjectCache
which functions like the PartialSnapshotCache but lives in the
ReadOnlySerializer rather than the StartupSerializer. These cache
entries are referenced using a new bytecode: ReadOnlyObjectCache. (To
make room for this, the ApiReference bytecode has been moved).

To reduce code duplication, the StartupSerializer has been refactored to
create a new base class RootSerializer, which ReadOnlySerializer also
subclasses. The base class is responsible primarily for keeping track of
already serialized roots and visiting the roots.

Cq-Include-Trybots: luci.chromium.try:linux_chromium_rel_ng
Change-Id: Iff26042886130ae22eccf2e11b35f6f226f4a792
Bug: v8:8191
Reviewed-on: https://chromium-review.googlesource.com/c/1244676
Commit-Queue: Dan Elphick <delphick@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Cr-Commit-Position: refs/heads/master@{#56681}