Add RTCRtpEncodingParameters.adaptivePtime to externs.

Currently available under origin trial in Chrome.
Documentation: https://w3c.github.io/webrtc-extensions/#dom-rtcrtpencodingparameters-adaptiveptime
PiperOrigin-RevId: 324556176

Author: Closure Team

externs/browser/w3c_rtc.js

@@ -867,6 +867,12 @@ RTCRtpEncodingParameters.prototype.rid;
  */
 RTCRtpEncodingParameters.prototype.scaleResolutionDownBy;
 
+/**
+ * @see https://w3c.github.io/webrtc-extensions/#dom-rtcrtpencodingparameters-adaptiveptime
+ * @type {?boolean|undefined}
+ */
+RTCRtpEncodingParameters.prototype.adaptivePtime;
+
 /**
  * @interface
  * @see https://www.w3.org/TR/webrtc/#rtcrtptransceiver-interface