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Similar to run-swarmed.py https://crrev.com/c/2623734,
"mb.py run -s" will use CAS instead of Isolate server.

Bug: 1163817
Change-Id: Ib0488417a1967736b0d665b49ed6f8d70e01cab6
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2626807
Auto-Submit: Junji Watanabe <jwata@google.com>
Reviewed-by: Takuto Ikuta <tikuta@chromium.org>
Reviewed-by: Dirk Pranke <dpranke@google.com>
Commit-Queue: Dirk Pranke <dpranke@google.com>
Cr-Commit-Position: refs/heads/master@{#843805}