- Changes OS::Allocate to first try an exact size aligned
  allocation, then padded allocations. All padded allocations should
  be trimmed.

Bug: chromium:800511
Change-Id: Iccab2eddbf2a3b08d2b83b95f96c766c9fad7a82
Reviewed-on: https://chromium-review.googlesource.com/875242Reviewed-by: Hannes Payer <hpayer@chromium.org>
Commit-Queue: Bill Budge <bbudge@chromium.org>
Cr-Commit-Position: refs/heads/master@{#50706}