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Add a new command line switch for ChromeDriver, to allow more
flexibility in controlling the log level.

Bug: chromedriver:2221
Change-Id: I788c56d823d7d63b9f65ce31105230d2ffa47887
Reviewed-on: https://chromium-review.googlesource.com/912589
Commit-Queue: John Chen <johnchen@chromium.org>
Reviewed-by: Jonathon Kereliuk <kereliuk@chromium.org>
Cr-Commit-Position: refs/heads/master@{#539539}