Add a deprecation message for multiple shadow roots.

Source/core/frame/UseCounter.cpp

Index: Source/core/frame/UseCounter.cpp
diff --git a/Source/core/frame/UseCounter.cpp b/Source/core/frame/UseCounter.cpp
index 33bafef013b2aea137be946057d0acf01259e2fc..2f464a3333e70606ca2d0a175da2bf5c54f135d2 100644
--- a/Source/core/frame/UseCounter.cpp
+++ b/Source/core/frame/UseCounter.cpp
@@ -900,6 +900,12 @@ String UseCounter::deprecationMessage(Feature feature)
     case DocumentGetCSSCanvasContext:
         return "The -webkit-canvas CSS feature is deprecated. Please use a positioned <canvas> element instead.";
 
+    case ElementCreateShadowRootMultiple:
+        return "Calling Element.createShadowRoot() for an element which already hosts a shadow root is deprecated. See https://www.chromestatus.com/features/4668884095336448 for more details.";
+
+    case ElementCreateShadowRootMultipleWithUserAgentShadowRoot:
+        return "Calling Element.createShadowRoot() for an element which already hosts a user-agent shadow root is deprecated. See https://www.chromestatus.com/features/4668884095336448 for more details.";
+
     // Features that aren't deprecated don't have a deprecation message.
     default:
         return String();