Implements the Allocate, Add, and HasKey operations. Also, adds GC
support for this new instance type.

Bug: v8:6443
Change-Id: I1cc7ba2faead2a11f7b0381a57858629e123aee6
Reviewed-on: https://chromium-review.googlesource.com/500447
Commit-Queue: Sathya Gunasekaran <gsathya@chromium.org>
Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Reviewed-by: Hannes Payer <hpayer@chromium.org>
Reviewed-by: Camillo Bruni <cbruni@chromium.org>
Cr-Commit-Position: refs/heads/master@{#45551}