- Update InitializeAllocationMemento to use newer CSA helper
- Fix AllocateJSArray to create AllocationMementos for empty arrays as well

Bug: v8:6211
Change-Id: I8731b04cdd500b877a54dee67f00f2899d91d86d
Reviewed-on: https://chromium-review.googlesource.com/566810
Commit-Queue: Camillo Bruni <cbruni@chromium.org>
Reviewed-by: Igor Sheludko <ishell@chromium.org>
Cr-Commit-Position: refs/heads/master@{#46594}