Additionally:
- Remove partiality from ContextRef::previous as long
  as we don't need it.
- Fix a nasty bug in serialization dispatch (the order
  of types was incorrect).

Bug: v8:7790
Change-Id: I354a69cf37e1dcdd691aab8af668c5cef165cf1e
Reviewed-on: https://chromium-review.googlesource.com/1224438Reviewed-by: Jaroslav Sevcik <jarin@chromium.org>
Reviewed-by: Maya Lekova <mslekova@chromium.org>
Commit-Queue: Georg Neis <neis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#55889}