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This is a reland of c5126022

TBR=dgozman@chromium.org

Original change's description:
> DevTools: Do not start tracing in renderer process explicitly.
>
> 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}

Change-Id: Iaa5195808a2339c1b3a52e122b906134fb859dc6
Reviewed-on: https://chromium-review.googlesource.com/1183824
Commit-Queue: Alexei Filippov <alph@chromium.org>
Reviewed-by: Alexei Filippov <alph@chromium.org>
Cr-Commit-Position: refs/heads/master@{#584952}