As the hook runs, we'll now be printing each trace's
progress. This uses some tricks with Python's subprocess.

Bug: angleproject:5210
Change-Id: I4e776a89c2b3e06ba67f9a82969aa394b24208d5
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2491104
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Tim Van Patten <timvp@google.com>
Reviewed-by: Cody Northrop <cnorthrop@google.com>