Support the monitoring mode in about:tracing
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: bf5cd6ccde378db821296732a091cfbcf5285fbd@164571 bbb929c8-8fbe-4397-9dbb-9b2b20218538

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