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 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> Change-Id: I0b4f3d55ea1799e35c9799c221f7129233f30b24 Reviewed-on: https://chromium-review.googlesource.com/c/1492972