- Adds a DCHECK to AllocatePages to enforce hint address alignment.
- Adds a DCHECK to AllocatePages to make length a multiple of allocation
  granularity.
- Properly aligns the hint address in MemoryAllocator::AllocateChunk
  to MemoryChunk::kAlignment.

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

Bug:chromium:756050

Change-Id: Ib991fa80609834107829a9559f4b5b6b0996fc72
Reviewed-on: https://chromium-review.googlesource.com/898095Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Commit-Queue: Bill Budge <bbudge@chromium.org>
Cr-Commit-Position: refs/heads/master@{#51065}