Support the monitoring mode in about:tracing

This is a corresponding change to trace_event.h in https://codereview.chromium.org/109933006/

Review URL: https://codereview.chromium.org/121053002

git-svn-id: svn://svn.chromium.org/blink/trunk@164571 bbb929c8-8fbe-4397-9dbb-9b2b20218538