Add a use counter for 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 399 matching lines @@
 
     case UseCounter::CSSSelectorInternalMediaControlsTextTrackList:
     case UseCounter::CSSSelectorInternalMediaControlsTextTrackListItem:
     case UseCounter::CSSSelectorInternalMediaControlsTextTrackListItemInput:
     case UseCounter::CSSSelectorInternalMediaControlsTextTrackListKindCaptions:
     case UseCounter::CSSSelectorInternalMediaControlsTextTrackListKindSubtitles:
       return willBeRemoved(
           "-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();
   }
 }
 
 }  // namespace blink