We need to be able to serialize a NativeModule, which is not bound to
any Isolate. Hence we should not want to pass any Isolate to the
serializer. This CL removes the dependence by not using the
ExternalReferenceTable from the Isolate, but instead using its own
ExternalReferenceList for serialization and deserialization. This
ExternalReferenceList only contains isolate-independent external
references.

R=mstarzinger@chromium.org

Bug: chromium:912043, chromium:912031
Change-Id: Iea5abd95dce9c54e618255cc577b6b43f002ac5d
Reviewed-on: https://chromium-review.googlesource.com/c/1363135
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
Cr-Commit-Position: refs/heads/master@{#58099}