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This is the standard way isolated scripts allow for callers to specify a
subset of tests to run. This CL adds this argument, and integrates it
with the existing method for specifying a subset of tests to run.

Bug: 533481
Change-Id: Ib63c18cfa5cb3db8a5256b7969dccb9541d4f9cc
Reviewed-on: https://chromium-review.googlesource.com/1112767
Commit-Queue: Stephen Martinis <martiniss@chromium.org>
Reviewed-by: Greg Thompson <grt@chromium.org>
Cr-Commit-Position: refs/heads/master@{#570182}