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- expose RunUnitTestsSerially and RunUnitTestsBatch
- hide private static ProcessTestResults in anonymous namespace

This will be used to implement ios-specific unit test
relaunch logic. Exposing the helpers makes it possible
to re-use them in ios code.

BUG=426870
R=sky@chromium.org

Review URL: https://codereview.chromium.org/879883002

Cr-Commit-Position: refs/heads/master@{#313476}