Context objects are allocated on the heap and thus should be Data
objects. This allows handling them through tracing in the GC through
the API.

Bug: chromium:1013149
Change-Id: Id3a7bfd57fab19a5669062ccf61c2f8588faf0bb
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2627307Reviewed-by: Camillo Bruni <cbruni@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#72120}