This CL removes PPC specific constants and makes the regular heap
object size limit a power of two.

Bug: chromium:852420
Change-Id: I5f4000f76d029d689d22112877eea124774b29d2
Reviewed-on: https://chromium-review.googlesource.com/c/1496367Reviewed-by: Hannes Payer <hpayer@chromium.org>
Commit-Queue: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#60011}