Implement TracingController::{Enable,Disable,Capture}Monitoring

This CL implements TracingController::EnableMonitoring,
TracingController::DisableMonitoring and
TracingController::CaptureMonitoringSnapshot.

BUG=241743
TEST=base_unittests::TraceEventTestFixture.TraceContinuousSampling,
content_browsertests::TracingControllerTest.EnableCaptureAndDisableMonitoring

Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=226701

R=dsinclair@chromium.org, joi@chromium.org, nduca@chromium.org, tsepez@chromium.org

Review URL: https://codereview.chromium.org/23531042

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@227262 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 227262