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