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The tracing is now initiated by the browser process. There's no need
to start it in renderer anymore. TracingStartedInPage event is not
going to be produced anymore. It has been replaced with TracingStartedInBrowser.

Change-Id: I9e7759f7f792421d3f32fd21e82066e13a4901dc
Reviewed-on: https://chromium-review.googlesource.com/1182287
Commit-Queue: Alexei Filippov <alph@chromium.org>
Reviewed-by: Dmitry Gozman <dgozman@chromium.org>
Cr-Commit-Position: refs/heads/master@{#584797}