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 EndTraceAndFlush() {
    num_flush_callbacks_ = 0;
    WaitableEvent flush_complete_event(false, false);
    EndTraceAndFlushAsync(&flush_complete_event);
    flush_complete_event.Wait();
  }

  void EndTraceAndFlush() {
    num_flush_callbacks_ = 0;
    WaitableEvent flush_complete_event(false, false);
    EndTraceAndFlushAsync(&flush_complete_event);
    flush_complete_event.Wait();
  }
