This CL makes it easier to organize the Context classes hierarchy and
simplifies Context class definition.

Bug: v8:8238
Change-Id: I65b8255daf255649c597dc195edf436d9471e3ea
Reviewed-on: https://chromium-review.googlesource.com/c/1350109
Commit-Queue: Igor Sheludko <ishell@chromium.org>
Reviewed-by: Toon Verwaest <verwaest@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Reviewed-by: Benedikt Meurer <bmeurer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#57848}