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With this change, scripts without precondition are not treated as
invalid, but runnable. Also includes a small test
ProtocolUtils::ParseScripts.

Bug: 806868
Change-Id: I6c286b1622bfeb4d635310396cb3b158ffd6933e
Reviewed-on: https://chromium-review.googlesource.com/1219748Reviewed-by: Ganggui Tang <gogerald@chromium.org>
Commit-Queue: Stephane Zermatten <szermatt@chromium.org>
Cr-Commit-Position: refs/heads/master@{#590333}