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rather than the TracingController.

This is in preparation for being able to handle proto traces
in addition to JSON.

R=ssid@chromium.org
BUG=925151

Change-Id: I91ed3c9648587fb19a5102b869a5d5ab688e0187
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1546146
Commit-Queue: oysteine <oysteine@chromium.org>
Reviewed-by: Avi Drissman <avi@chromium.org>
Reviewed-by: ssid <ssid@chromium.org>
Cr-Commit-Position: refs/heads/master@{#657413}