This patch changes the backing store of slow properties to be a
new instance type called PropertyArray.

Currently the only difference between this and a FixedArray is
the map. A future patch will change the length property to store
the hash code.

Bug: v8:5717, v8:6404
Cq-Include-Trybots: master.tryserver.chromium.linux:linux_chromium_rel_ng
Change-Id: Iaebc98f42e6d93c1392772e6f837787beb64afec
Reviewed-on: https://chromium-review.googlesource.com/539028Reviewed-by: Jakob Kummerow <jkummerow@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
Reviewed-by: Camillo Bruni <cbruni@chromium.org>
Commit-Queue: Sathya Gunasekaran <gsathya@chromium.org>
Cr-Commit-Position: refs/heads/master@{#46569}