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This also rewrites some of the args logic in generate_buildbot_json.py
and allows for callers to supply additional keys containing args they'd
like to pull in (e.g. "gtest_args" for args that should apply only to
gtests).

Bug: 878321
Change-Id: I2314c90d5959f3cfdaa3898a2de1af2331d07002
Reviewed-on: https://chromium-review.googlesource.com/1197407Reviewed-by: Kenneth Russell <kbr@chromium.org>
Reviewed-by: Stephen Martinis <martiniss@chromium.org>
Commit-Queue: John Budorick <jbudorick@chromium.org>
Cr-Commit-Position: refs/heads/master@{#588219}