This CL:
  * Ensures that no allocator shim state is carried over between tests.
  * Updates malloc zone interception logic to reflect the fact that the default
  malloc zone is not included in malloc_get_all_zones.
  * Turn MallocZoneFunctions into POD. Update g_malloc_zones to be an array
  rather than a pointer.

BUG=693237

Review-Url: https://codereview.chromium.org/2727463002
Cr-Commit-Position: refs/heads/master@{#459964}