message: Remove has_slots from a class that does not need it
  sigslot::has_slots<> is only needed in the class that listens to
  signals, not the class that sends it.
  Cr-Commit-Position: refs/heads/master@{#32574}
diff: @@ -37,8 +37,7 @@ namespace webrtc {
  // DTLS certificate request callback class.
  class WebRtcCertificateGeneratorCallback
  - : public rtc::RTCCertificateGeneratorCallback,
  - public sigslot::has_slots<> {
  + : public rtc::RTCCertificateGeneratorCallback {
  public:
  // |rtc::RTCCertificateGeneratorCallback| overrides.
  void OnSuccess(
  