Add a deprecation message to rtcpMuxPolicy of negotiate.

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

 // Copyright 2016 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "core/frame/Deprecation.h"
 
 #include "core/dom/Document.h"
 #include "core/dom/ExecutionContext.h"
 #include "core/frame/FrameConsole.h"
 #include "core/frame/FrameHost.h"
@@ skipping 412 matching lines @@
              "https://www.chromestatus.com/features/6680566019653632 for more "
              "details.";
 
     case UseCounter::SubtleCryptoOnlyStrictSecureContextCheckFailed:
       return String::format(
           "Web Crypto API usage inside secure frames with non-secure ancestors "
           "is deprecated. The API will no longer be exposed in these contexts "
           "as of %s. See https://www.chromestatus.com/features/5030265697075200"
           " for more details.", milestoneString(M59));
 
+    case UseCounter::RtcpMuxPolicyNegotiate:
+      return String::format(
+          "The rtcpMuxPolicy option is being considered for "
+          "removal and may be removed no earlier than %s. If you depend on it, "
+          "please see https://www.chromestatus.com/features/5654810086866944 "
+          "for more details.",
+          milestoneString(M60));
+
     // Features that aren't deprecated don't have a deprecation message.
     default:
       return String();
   }
 }
 
 }  // namespace blink