Clean up the PoolAllocator class Many things were unnecessarily in the header and are now moved to the source file. Bug: angleproject:3167 Change-Id: I52b83fdde5b6e984c5f5491c2ce050debf78f8b8 Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/3206072 Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org> Reviewed-by: Jamie Madill <jmadill@chromium.org> Reviewed-by: Tim Van Patten <timvp@google.com>