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 f43775ef8786494f1dd7ecf39eb99ff3ed3f37ed..2be9b55f4f7027acd0eb7b817f9cca4849c39947 100644
--- a/third_party/WebKit/Source/modules/peerconnection/RTCPeerConnection.cpp
+++ b/third_party/WebKit/Source/modules/peerconnection/RTCPeerConnection.cpp
@@ -263,6 +263,7 @@ WebRTCConfiguration parseConfiguration(ExecutionContext* context,
   String rtcpMuxPolicyString = configuration.rtcpMuxPolicy();
   if (rtcpMuxPolicyString == "negotiate") {
     rtcpMuxPolicy = WebRTCRtcpMuxPolicy::kNegotiate;
+    UseCounter::count(context, UseCounter::RtcpMuxPolicyNegotiate);
   } else {
     DCHECK_EQ(rtcpMuxPolicyString, "require");
   }