New API is here: https://github.com/tc39/proposal-weakrefs/issues/55

The WeakCell parts stay in the old API, resulting in temporary code duplication
in some parts. Those parts will go away once the WeakCell-related parts are
migrated to the new API (but the spec needs some work first).

BUG=v8:8179

Change-Id: I81ca824a14d830e3c5fa515d5ad7e5f78c10e19d
Reviewed-on: https://chromium-review.googlesource.com/c/1378171
Commit-Queue: Marja Hölttä <marja@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Reviewed-by: Sathya Gunasekaran <gsathya@chromium.org>
Cr-Commit-Position: refs/heads/master@{#58264}