Oilpan: Introduce memory pool for CallbackStacks

Once we enable per-thread heaps, we will end up with having CallbackStacks per thread.
To prevent the memory bloat, this CL introduces a memory pool for CallbackStacks
shared among all threads. 256 KB is pre-allocated for the memory pool.

BUG=

Review-Url: https://codereview.chromium.org/2127453002
Cr-Commit-Position: refs/heads/master@{#404381}
6 files changed