Using KeyAccumulator::GetKeys directly enables fast-paths by checking
if the enum-cache is set.

Drive-by-fix:
- Reduce public interface of KeyAccumulator to prevent these
  performance issues in the future.
- Fix value-serializer.cc includes

Change-Id: I2cc7b3bf9d1e42e699829427163ecbdee92c9007
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2520898
Commit-Queue: Camillo Bruni <cbruni@chromium.org>
Reviewed-by: Igor Sheludko <ishell@chromium.org>
Cr-Commit-Position: refs/heads/master@{#71050}