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Passing --trace-to-file on the command line will trace a default set of
categories to a default trace file name (trace.json).
--trace-to-file=categories --trace-to-file-name=somefile.json allow for
more configuration.

BUG=none

Review URL: https://codereview.chromium.org/542893002

Cr-Commit-Position: refs/heads/master@{#296770}