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Macs all have SSDs, so generating ssd:0 for builderless mac builders
forces the definition to also specify ssd=None.

Change-Id: I61b09dab79e9ec37867c6d24c39982f360083a5d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2368322Reviewed-by: Stephen Martinis <martiniss@chromium.org>
Commit-Queue: Garrett Beaty <gbeaty@chromium.org>
Cr-Commit-Position: refs/heads/master@{#801475}