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..d675f40d50cd0a25ac692d26cd57e36bd9dfcb92 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 an user-agent shadow root is deprecated. See https://www.chromestatus.com/features/4668884095336448 for more details.";

[kochi, 2015/05/28 04:19:44]

nit: "a" user-agent

[hayato, 2015/05/28 11:55:41]

Done.

+
     // Features that aren't deprecated don't have a deprecation message.
     default:
         return String();