This ensures that MigrateFastToFast does not overflow the length of the
property array.

Bug: chromium:789393
Change-Id: I77adc319c1c8c469ea482bad35ead8661d535192
Reviewed-on: https://chromium-review.googlesource.com/824167
Commit-Queue: Ulan Degenbaev <ulan@chromium.org>
Reviewed-by: Igor Sheludko <ishell@chromium.org>
Reviewed-by: Sathya Gunasekaran <gsathya@chromium.org>
Cr-Commit-Position: refs/heads/master@{#50086}