Move thread pool classes to common/ In preparation for access by image_util files. Bug: b/250688943 Change-Id: I24777269a5071eae9a60f939635d01ed7246461f Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/3961454 Reviewed-by: Jamie Madill <jmadill@chromium.org> Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>