The roundTripTime is one of the metrics that should be exposed based on the RTCP Receiver Report which would be very valuable to getStats() users.
This is part of the RTCRemoteInboundRtpStreamStats dictionary which is currently not implemented in spec-compliant getStats()[1].

This is the dictionary + hierarchy, see spec[2].

dictionary RTCRtpStreamStats : RTCStats {
             unsigned long ssrc;
             DOMString kind;
             DOMString transportId;
             DOMString codecId;
             unsigned long firCount;
             unsigned long pliCount;
             unsigned long nackCount;
             unsigned long sliCount;
             unsigned long long qpSum;
};

dictionary RTCReceivedRtpStreamStats : RTCRtpStreamStats {
             unsigned long packetsReceived;
             long packetsLost;
             double jitter;
             unsigned long packetsDiscarded;
             unsigned long packetsRepaired;
             unsigned long burstPacketsLost;
             unsigned long burstPacketsDiscarded;
             unsigned long burstLossCount;
             unsigned long burstDiscardCount;
             double burstLossRate;
             double burstDiscardRate;
             double gapLossRate;
             double gapDiscardRate;
};

dictionary RTCRemoteInboundRtpStreamStats : RTCReceivedRtpStreamStats {
             DOMString localId;
             double roundTripTime;
             double fractionLost;
};

The current dictionary implementation[3] only has inbound and outbound RTP, so the hierarchy is not up-to-date (no RTC[Sender/Receiver]RtpStreamStats) but the final dictionary contains the same members.

[1] https://cs.chromium.org/chromium/src/third_party/webrtc/pc/rtcstatscollector.h?q=rtcstatscollector.h&sq=package:chromium&dr=CSs&l=11
[2] https://w3c.github.io/webrtc-stats/#remoteinboundrtpstats-dict*
[3] https://cs.chromium.org/chromium/src/third_party/webrtc/api/stats/rtcstats_objects.h?l=326