Vulkan:Adding custom pool allocator 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/1476953 Reviewed-by: Jamie Madill <jmadill@google.com> Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org> Commit-Queue: Tobin Ehlis <tobine@google.com>