- Implements GetPageAllocator and OnCriticalMemoryPressure methods, delegating
  to the default platform, in preparation for switching over to these for
  handling memory management.

Bug: chromium:756050
Change-Id: I66edb35ce2669ee9ffd53e82a63b598cc5b7f40c
Reviewed-on: https://chromium-review.googlesource.com/845035
Commit-Queue: Bill Budge <bbudge@chromium.org>
Reviewed-by: Eric Holk <eholk@chromium.org>
Cr-Commit-Position: refs/heads/master@{#50330}