Tracing: Add support for PII whitelisting of individual trace event arguments

R=dsinclair,shatch
BUG=546093

Review URL: https://codereview.chromium.org/1415013003

Cr-Commit-Position: refs/heads/master@{#356690}
void BlockUntilStopped(WaitableEvent* task_start_event,
                       WaitableEvent* task_stop_event) {
  task_start_event->Signal();
  task_stop_event->Wait();
}

void BlockUntilStopped(WaitableEvent* task_start_event,
                       WaitableEvent* task_stop_event) {
  task_start_event->Signal();
  task_stop_event->Wait();
}
