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/1476953Reviewed-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
