DevTools: use InspectorClient methods to enable/disable tracing

Instead of sending Tracing.started/Tracing.stopped events which are intercepted in the browser explicit API is used for enabling/disabling trace event recording.

BUG=398787

Review URL: https://codereview.chromium.org/514633003

git-svn-id: svn://svn.chromium.org/blink/trunk@181029 bbb929c8-8fbe-4397-9dbb-9b2b20218538