message: Make Clock::ConvertTimestampToNtpTime pure virtual
  No-Try because of lack of infra lack of capacity on macs.
  No-Try: True
  Cr-Commit-Position: refs/heads/master@{#34073}
diff: @@ -47,11 +47,7 @@ class RTC_EXPORT Clock {
  int64_t CurrentNtpInMilliseconds() { return CurrentNtpTime().ToMs(); }
  // Converts between a relative timestamp returned by this clock, to NTP time.
  - // TODO(bugs.webrtc.org/11327): Make this method pure virtual,
  - // and delete default dummy implementation.
  - virtual NtpTime ConvertTimestampToNtpTime(Timestamp timestamp) {
  - return NtpTime();
  - }
  + virtual NtpTime ConvertTimestampToNtpTime(Timestamp timestamp) = 0;
  int64_t ConvertTimestampToNtpTimeInMilliseconds(int64_t timestamp_ms) {
  return ConvertTimestampToNtpTime(Timestamp::Millis(timestamp_ms)).ToMs();
  }
  