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I've joined the current mb_config.pyl
with /src/cr-buildbucket.cfg to remap masters to buckets for a new
bucket-based mb_config_buckets.pyl.
Also the old logic that used the 'master: { chromium: {...}}' dict to
specify the chromium builders that blacklist chrome proprietary mixins
now looks at a top-level config key in mb_config_buckets.pyl
called 'public_artifact_builders'.

mb.py has been modified to support the new type of mapping in
mb_config_buckets.pyl and will use it by default if a bucket is specified.
I hope to move all the code that uses mb to use buckets instead of masters,
at which point I plan to remove all the master-related functions from mb.py
The new functions have been written in a way that makes it easy to remove
the old master-based functions at the cost of being less DRY.
Validation stuff was factored out to a helper file to lighten the load
inside of mb.py.
Also there seems to be a dead function which I removed: CheckCompile.

This pyl also excludes all the internal builders, which will be
moved to src-internal.

Bug: 1028719
Change-Id: I86b55b12def06afab81faec66463963ede2f3244
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1963441
Commit-Queue: Gregory Guterman <guterman@google.com>
Reviewed-by: Aaron Gable <agable@chromium.org>
Reviewed-by: Stephen Martinis <martiniss@chromium.org>
Cr-Commit-Position: refs/heads/master@{#731713}