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 422 matching lines @@
     case UseCounter::FileReaderSyncInServiceWorker:
       return willBeRemoved("FileReaderSync in service workers", M59,
                            "5739144722513920");
 
     case UseCounter::SelectionAddRangeIntersect:
       return willBeRemoved(
           "The behavior that Selection.addRange() merges existing Range and "
           "the specified Range",
           M58, "6680566019653632");
 
+    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