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DevTools: use explicit IPC messages for enabling/disabling tracing instead of intercepting protocol events

Added IPC plumbing that allows InspectorTracingAgent to enable/disable  browser-wide tracing. It was implemented by intercepting Tracing.started/Tracing.stopped events in the browser. Tracing.stopped event in the DevTools protocol was used by the renderer to notify the browser that it should stop trace event recording and it didn't make much sense for the client as there is already Tracing.tracingComplete event in the protocol.

BUG=398787

Review URL: https://codereview.chromium.org/511873002

Cr-Commit-Position: refs/heads/master@{#292379}