- Add the WeakRef class and its deref() function.

- Add WeakFactory.prototype.makeRef

- Implement the "keep during job" behavior for WeakRef constructor and deref().

- Here we keep the targets alive longer than until the end of the job
  (microtask), contradicting the spec. However, this is probably the indended
  behavior, see https://github.com/tc39/proposal-weakrefs/issues/39 .

BUG=v8:8179

Change-Id: I41990d41ac1799e34f675d8431b9a7aa7ed3d48d
Reviewed-on: https://chromium-review.googlesource.com/c/1306435
Commit-Queue: Marja Hölttä <marja@chromium.org>
Reviewed-by: Benedikt Meurer <bmeurer@chromium.org>
Reviewed-by: Sathya Gunasekaran <gsathya@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#57242}