Add stats.atSendCapacity

index.bs

@@ -1494,7 +1494,8 @@ dictionary WebTransportConnectionStats {
   required DOMHighResTimeStamp rttVariation;
   required DOMHighResTimeStamp minRtt;
   required WebTransportDatagramStats datagrams;
-  required unsigned long long? estimatedSendRate;
+  required unsigned long long? estimatedSendRate = null;
+  required boolean isSendDataLimited = true;
 };
 </pre>
 
@@ -1532,7 +1533,14 @@ The dictionary SHALL have the following attributes:
    {{WebTransport/congestionControl}}). If the user agent does not
    currently have an estimate, the member MUST be the `null` value.
    The member can be `null` even if it was not `null` in previous results.
-
+: <dfn for="WebTransportConnectionStats" dict-member>isSendDataLimited</dfn>
+:: False if the rate at which queued data will be sent by the user agent on all streams
+   and datagrams that share a [=WebTransport session=], as calculated by the congestion
+   control algorithm, is not currently application limited; Otherwise true.
+   <div class="note">
+   When {{isSendDataLimited}} is `false`, the {{estimatedSendRate}} reflects a ceiling.
+   However, {{estimatedSendRate}} can be `null` while {{isSendDataLimited}} is true.
+   <div>
 
 ## `WebTransportDatagramStats` Dictionary ##  {#web-transport-datagram-stats}