Merging to M57 branch 2987.

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 85a06f66c4fa2848a49ca3112530b0a66fdaf5de..ab82fae2e1a4897d4b1a8af801b9ad939d3e432e 100644
--- a/third_party/WebKit/Source/core/frame/Deprecation.cpp
+++ b/third_party/WebKit/Source/core/frame/Deprecation.cpp
@@ -440,6 +440,14 @@ String Deprecation::deprecationMessage(UseCounter::Feature feature) {
           "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();