Add a use counter for RtcpMuxPolicy of "negotiate".

third_party/WebKit/Source/core/frame/Deprecation.cpp

Index: third_party/WebKit/Source/core/frame/Deprecation.cpp
diff --git a/third_party/WebKit/Source/core/frame/Deprecation.cpp b/third_party/WebKit/Source/core/frame/Deprecation.cpp
index 81f66ffe13713b57cee662ae4f766601703474de..3c667f16b97903cdde81c6a884ad9f948db4e754 100644
--- a/third_party/WebKit/Source/core/frame/Deprecation.cpp
+++ b/third_party/WebKit/Source/core/frame/Deprecation.cpp
@@ -417,6 +417,9 @@ String Deprecation::deprecationMessage(UseCounter::Feature feature) {
           "-internal-media-controls-text-track-list* selectors", M59,
           "5661431349379072");
 
+    case UseCounter::RtcpMuxPolicyNegotiate:
+      return replacedBy("rtcpMuxPolicy \"negotiate\"", "\"require\"");

[foolip, 2017/02/15 06:46:36]

If the "negotiate" value is removed, I think we'd end up removing the
rtcpMuxPolicy stuff entirely, rather than keeping it around with just one
allowed value.

+
     // Features that aren't deprecated don't have a deprecation message.
     default:
       return String();