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..c207256565d3dc4508f065bab10bc36ab00f2bf0 100644
--- a/third_party/WebKit/Source/core/frame/Deprecation.cpp
+++ b/third_party/WebKit/Source/core/frame/Deprecation.cpp
@@ -417,6 +417,10 @@ String Deprecation::deprecationMessage(UseCounter::Feature feature) {
           "-internal-media-controls-text-track-list* selectors", M59,
           "5661431349379072");
 
+    case UseCounter::RtcpMuxPolicyNegotiate:
+      return "The rtcpMuxPolicy \"negotiate\" will be deprecated. Please use "

[Taylor_Brandstetter, 2017/02/09 00:29:43]

Should change to "is deprecated"? I believe saying something "is deprecated" is
equivalent to saying "is discouraged, may be removed in the future".

[zhihuang1, 2017/02/09 01:46:07]

By applying the template, the message is now 
"
The rtcpMuxPolicy "negotiate" is deprecated. Please use "require" instead.
"

+             "\"require\" instead if possible.";
+
     // Features that aren't deprecated don't have a deprecation message.
     default:
       return String();