Add luci-go and mb to DEPS.

These tools are used by scripts/trigger.py and by the src-side testing.
Also adds .vpython and .vpython3 spec files which can't be easily
auto-rolled from the Chromium src/ repo.

Bug: angleproject:5040
Change-Id: Ie6384c176b391dae4fe19582ec59c2c1d6ec22d8
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2408717
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Yuly Novikov <ynovikov@chromium.org>

.vpython3

@@ -0,0 +1,32 @@
+# This is a vpython "spec" file.
+#
+# It describes patterns for python wheel dependencies of the python scripts in
+# the chromium repo, particularly for dependencies that have compiled components
+# (since pure-python dependencies can be easily vendored into third_party).
+#
+# When vpython is invoked, it finds this file and builds a python VirtualEnv,
+# containing all of the dependencies described in this file, fetching them from
+# CIPD (the "Chrome Infrastructure Package Deployer" service). Unlike `pip`,
+# this never requires the end-user machine to have a working python extension
+# compilation environment. All of these packages are built using:
+#   https://chromium.googlesource.com/infra/infra/+/master/infra/tools/dockerbuild/
+#
+# All python scripts in the repo share this same spec, to avoid dependency
+# fragmentation.
+#
+# If you have depot_tools installed in your $PATH, you can invoke python scripts
+# in this repo by running them as you normally would run them, except
+# substituting `vpython` instead of `python` on the command line, e.g.:
+#   vpython path/to/script.py some --arguments
+#
+# Read more about `vpython` and how to modify this file here:
+#   https://chromium.googlesource.com/infra/infra/+/master/doc/users/vpython.md
+
+python_version: "3.8"
+
+# TODO(https://crbug.com/898348): Add in necessary wheels as Python3 versions
+# become available.
+wheel: <
+  name: "infra/python/wheels/six-py2_py3"
+  version: "version:1.10.0"
+>