Adds concurrency support for:
- MemoryChunk: Fragmentation counters
- MemoryChunk: High-water mark
- MemoryAllocator: Lowest and highest ever allocated addresses, size, and
  capacity

R=hpayer@chromium.org
BUG=chromium:524425
LOG=N

This reverts commit 0db34dbe.

BUG=

Review URL: https://codereview.chromium.org/1346973002

Cr-Commit-Position: refs/heads/master@{#30760}