Add a use counter for RtcpMuxPolicy of "negotiate".

third_party/WebKit/Source/modules/peerconnection/RTCPeerConnection.cpp

Index: third_party/WebKit/Source/modules/peerconnection/RTCPeerConnection.cpp
diff --git a/third_party/WebKit/Source/modules/peerconnection/RTCPeerConnection.cpp b/third_party/WebKit/Source/modules/peerconnection/RTCPeerConnection.cpp
index df9d88ec758b195311ddb78892e82de4f0d8d9d7..35ced13ad81406021c0d799e68b97331cd8a39bf 100644
--- a/third_party/WebKit/Source/modules/peerconnection/RTCPeerConnection.cpp
+++ b/third_party/WebKit/Source/modules/peerconnection/RTCPeerConnection.cpp
@@ -264,6 +264,7 @@ WebRTCConfiguration parseConfiguration(ExecutionContext* context,
   String rtcpMuxPolicyString = configuration.rtcpMuxPolicy();
   if (rtcpMuxPolicyString == "negotiate") {
     rtcpMuxPolicy = WebRTCRtcpMuxPolicy::kNegotiate;
+    UseCounter::count(context, UseCounter::RtcpMuxPolicyNegotiate);
   } else {
     DCHECK_EQ(rtcpMuxPolicyString, "require");
   }