Measure the usage of prefixed Shadow DOM APIs.

Source/core/page/UseCounter.h

 /*
  * 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 128 matching lines @@
         XMLDocument,
         XSLProcessingInstruction,
         XSLTProcessor,
         SVGSwitchElement,
         PrefixedDocumentRegister,
         HTMLShadowElementOlderShadowRoot,
         DocumentAll,
         FormElement,
         DemotedFormElement,
         CaptureAttributeAsEnum,
+        ShadowDOMPrefixedPseudo,
+        ShadowDOMPrefixedCreateShadowRoot,
+        ShadowDOMPrefixedShadowRoot,
         // Add new features immediately above this line. Don't change assigned numbers of each items.
         NumberOfFeatures, // This enum value must be last.
     };
 
     // "count" sets the bit for this feature to 1. Repeated calls are ignored.
     static void count(Document*, Feature);
     static void count(DOMWindow*, Feature);
     void count(CSSPropertyID);
 
     // "countDeprecation" sets the bit for this feature to 1, and sends a deprecation
@@ skipping 31 matching lines @@
 
     void updateMeasurements();
 
     OwnPtr<BitVector> m_countBits;
     BitVector m_CSSFeatureBits;
 };
 
 } // namespace WebCore
 
 #endif // UseCounter_h