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}
  bool TraceHasMatchingString(const char* str) {
    return last_file_contents_.find(str) != std::string::npos;
  }

  bool TraceHasMatchingString(const char* str) {
    return last_file_contents_.find(str) != std::string::npos;
  }
