Migrated pool allocator used by compiler to common.

Planning to use this for ANGLE custom command buffers so this some
refactoring in preparation for that work.

Added a unit test to check PoolAllocator functionality.

Bug: angleproject:2951
Change-Id: I29618cfdb065b8a5fefd40719a35d27b1f6e99ef
Reviewed-on: https://chromium-review.googlesource.com/c/1476953Reviewed-by: Jamie Madill <jmadill@google.com>
Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>
Commit-Queue: Tobin Ehlis <tobine@google.com>