Add a deprecation message for multiple shadow roots.

Source/core/frame/UseCounter.cpp

 /*
  * Copyright (C) 2012 Google, Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
  * are met:
  * 1. Redistributions of source code must retain the above copyright
  *    notice, this list of conditions and the following disclaimer.
  * 2. Redistributions in binary form must reproduce the above copyright
  *    notice, this list of conditions and the following disclaimer in the
@@ skipping 882 matching lines @@
 
     case EncryptedMediaInsecureOrigin:
         return "requestMediaKeySystemAccess() is deprecated on insecure origins in the specification. Support will be removed in the future. You should consider switching your application to a secure origin, such as HTTPS. See https://goo.gl/rStTGz for more details.";
 
     case PushSubscriptionId:
         return "'PushSubscription.subscriptionId' is deprecated and is now included in 'PushSubscription.endpoint'. It will be removed in Chrome 45, around August 2015.";
 
     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();
     }
 }
 
 void UseCounter::count(CSSParserContext context, CSSPropertyID feature)
 {
     ASSERT(feature >= firstCSSProperty);
     ASSERT(feature <= lastUnresolvedCSSProperty);
@@ skipping 27 matching lines @@
 UseCounter* UseCounter::getFrom(const StyleSheetContents* sheetContents)
 {
     // FIXME: We may want to handle stylesheets that have multiple owners
     //        http://crbug.com/242125
     if (sheetContents && sheetContents->hasSingleOwnerNode())
         return getFrom(sheetContents->singleOwnerDocument());
     return 0;
 }
 
 } // namespace blink