- Adds abstract base class PageAllocator, defined in v8-platform.h. Adds
  GetPageAllocator method to v8::Platform.
- Implements a DefaultPageAllocator, implemented in terms of base::OS
  page allocation methods.

Bug: chromium:756050
Cq-Include-Trybots: master.tryserver.chromium.linux:linux_chromium_rel_ng
Change-Id: Iece0b261a07294a49c30ac25e848dc39cb1a32e2
Reviewed-on: https://chromium-review.googlesource.com/809778
Commit-Queue: Bill Budge <bbudge@chromium.org>
Reviewed-by: Hannes Payer <hpayer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#50282}