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Add win-1909 to os enum, update mb config.

Bug: 990885,1125612
Change-Id: I419a4ebf018d70841ec62b8bdf2e9dfd26e21cfd
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2399721Reviewed-by: Garrett Beaty <gbeaty@chromium.org>
Commit-Queue: Ian Struiksma <ianstruiksma@google.com>
Cr-Commit-Position: refs/heads/master@{#805403}