Merge GPU generate_buildbot_json.py into testing/buildbot version.

Redefine the chromium.gpu, chromium.gpu.fyi and client.v8.fyi
waterfalls and test suites in waterfalls.pyl and test_suites.pyl,
respectively.

The goal of this conversion was to generate these three waterfalls'
JSON files almost exactly as they currently are, which has been
achieved. There are however many cleanups that should be done to
simplify the test suites and the configurations of the GPU bots, which
will be done in follow-on CLs and bugs.

Added a few needed features to the testing/buildbot version of the
generator script. Covered these in the unit tests; the coverage test
continues to pass. Deleted the old GPU script and updated docs that
referred to it.

Tbr: dpranke@chromium.org
Tbr: jbudorick@chromium.org
Tbr: ynovikov@chromium.org
Tbr: geofflang@chromium.org
Tbr: jmadill@chromium.org
Bug: 844815
Cq-Include-Trybots: luci.chromium.try:android_optional_gpu_tests_rel;luci.chromium.try:linux_optional_gpu_tests_rel;luci.chromium.try:mac_optional_gpu_tests_rel;luci.chromium.try:win_optional_gpu_tests_rel
Change-Id: I4e472d0e55a700fe3cec5043dcd260d31de8fd41
Reviewed-on: https://chromium-review.googlesource.com/1067132
Reviewed-by: Kenneth Russell <kbr@chromium.org>
Commit-Queue: Kenneth Russell <kbr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#563939}
12 files changed