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All of chrome's mb_configs should have been swapped over to using
'builder_groups'.

V8's and WebRTC's copies of mb.py still have references to 'masters':
https://chromium.googlesource.com/v8/v8/+/master/tools/mb/mb.py
https://webrtc.googlesource.com/src/+/master/tools_webrtc/mb/mb.py

So updating those are left as a follow-up for the relevant project
owners.

Bug: 1117773
Change-Id: Ic58469c050cac77d834c63f4a2b6478c377d9411
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2622751Reviewed-by: Dirk Pranke <dpranke@google.com>
Commit-Queue: Ben Pastene <bpastene@chromium.org>
Cr-Commit-Position: refs/heads/master@{#843093}