Make ScriptPromiseProperty carry execution context directly

... as opposed to using inheritance from ExecutionContextClient.

Change-Id: I07bf6dcd818b80dc0f373dbc106a9054a7eaebe0
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5405475


Commit-Queue: Andrey Kosyakov <caseq@chromium.org>
Reviewed-by: Nate Chapin <japhet@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1280205}