Update index.bs

Author: tonyherre

index.bs

@@ -330,7 +330,8 @@ dictionary RTCEncodedVideoFrameMetadata {
     unsigned long temporalIndex;
     unsigned long synchronizationSource;
     octet payloadType;
-  sequence<unsigned long> contributingSources;
+    sequence<unsigned long> contributingSources;
+    DOMHighResTimeStamp captureTime;
 };
 </pre>
 
@@ -365,6 +366,21 @@ dictionary RTCEncodedVideoFrameMetadata {
             The list of contribution sources (csrc list) as defined in [[RFC3550]].
         </p>
     </dd>
+    <dt>
+        <dfn>captureTime</dfn> of type <span class=
+            "idlMemberType">DOMHighResTimeStamp</span>
+    </dt>
+    <dd>
+        <p>
+		For video frames coming from a local source, this is the time at which
+		the frame was captured by the camera.
+		For video frames coming from remote source, the capture time is based on the RTP
+		timestamp of the frame and estimated using clock synchronization.
+		This is best effort and can use methods like using RTCP SR as specified in
+		RFC 3550 Section 6.4.1, or by other alternative means if use by RTCP SR isn’t feasible.
+		As defined in VideoFrameCallbackMetadata.
+        </p>
+    </dd>
 </dl>