The only empty PropertyArray is the empty_property_array object on the
isolate. Allowing empty PropertyArrays causes the turbofan to ignore the
existing hash when growing the backing store again. We currently only end
up with the empty PropertyArray when following back transitions.

Bug: chromium:781218, chromium:783713
Change-Id: If41dd09b965cdc8d957b9ca50ba3c8a7f4254769
Reviewed-on: https://chromium-review.googlesource.com/763230
Commit-Queue: Camillo Bruni <cbruni@chromium.org>
Reviewed-by: Hannes Payer <hpayer@chromium.org>
Reviewed-by: Sathya Gunasekaran <gsathya@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Reviewed-by: Jakob Kummerow <jkummerow@chromium.org>
Cr-Commit-Position: refs/heads/master@{#49318}