The generate_test_spec_json.py file now reads ANGLE's waterfalls.pyl
and Chromium's mixins.pyl to find the referenced mixins and copy them
into ANGLE's mixins.pyl. This way we can update ANGLE's mixins easily
without needing to manually copy them from Chromium's. In a follow-up
we could make this run on the Chromium auto-roller.

Bug: angleproject:5114
Change-Id: I332b3182732985b261e107dfe9a5c7c413c410a0
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2849184Reviewed-by: Yuly Novikov <ynovikov@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>