A new map wich references its keys weakly is added to v8.h. Internally it uses the same storage as JSWeakMap but doesn't depend on the JavaScript part of WeakMap implementation in weak-collection.js, hence it can be instantiated without entering any context.

BUG=chromium:437416
LOG=Y

Review URL: https://codereview.chromium.org/891473005

Cr-Commit-Position: refs/heads/master@{#26425}