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This CL implements the data structures that are needed for bookkeeping
V8 contexts, and related concepts. The lifetime management of these
structures is testing in isolation.

A further CL will implement business logic on V8ContextTracker, and add
tests for that.

BUG=1085129

Change-Id: Iececc631fa38f04359ebeb823c53b641e57a4f24
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2443490
Commit-Queue: Chris Hamilton <chrisha@chromium.org>
Reviewed-by: Joe Mason <joenotcharles@chromium.org>
Cr-Commit-Position: refs/heads/master@{#813266}