This adds ExecutionContextDescription.uniqueId for a system-unique
way to identify an execution context and supports it in Runtime.evaluate.
This allows a client to avoid accidentally executing an expression
in a context different from that originally intended if a navigation
occurs while Runtime.evaluate is in flight.

Design doc: https://docs.google.com/document/d/1vGVWvKP9FTTX6kimcUJR_PAfVgDeIzXXITFpl0SyghQ

Bug: v8:11268, chromium:1101897
Change-Id: I4c6bec562ffc85312559316f639d641780144039
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2594538
Commit-Queue: Andrey Kosyakov <caseq@chromium.org>
Reviewed-by: Dmitry Gozman <dgozman@chromium.org>
Reviewed-by: Benedikt Meurer <bmeurer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#71869}