- Adds a DCHECK to FreePages that size is a multiple of allocation
  granularity.
- Makes VirtualMemory::Free conform to this.

This is to conform more closely to Chromium's page allocator API.

Bug:chromium:756050

Change-Id: I673e1c225b8bd1009775de1597b575120bd06f8e
Reviewed-on: https://chromium-review.googlesource.com/898008Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Commit-Queue: Bill Budge <bbudge@chromium.org>
Cr-Commit-Position: refs/heads/master@{#51064}