Add a UseCounter for clip CSS or -webkit-clip-path that clips positioned descendants.

Source/core/frame/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 819 matching lines @@
         PresentationSessionSend = 934,
         PresentationSessionStateChangeEventListener = 935,
         PresentationSessionMessageEventListener = 936,
         CSSAnimationsStackedNeutralKeyframe = 937,
         ReadingCheckedInClickHandler = 938,
         FlexboxIntrinsicSizeAlgorithmIsDifferent = 939,
         HTMLImportsHasStyleSheets = 940,
         WebkitTextInClipProperty = 941,
         WebkitTextInColorProperty = 942,
         HeaderValueNotMatchingRFC7230 = 943,
+        ClipPathOfPositionedElement = 944,
+        ClipCssOfPositionedElement = 945,
 
         // Add new features immediately above this line. Don't change assigned
         // numbers of any item, and don't reuse removed slots.
         // Also, run update_use_counter_feature_enum.py in chromium/src/tools/metrics/histograms/
         // to update the UMA mapping.
         NumberOfFeatures, // This enum value must be last.
     };
 
     // "count" sets the bit for this feature to 1. Repeated calls are ignored.
     static void count(const Frame*, Feature);
@@ skipping 84 matching lines @@
 
     bool hasRecordedMeasurement(Feature feature) const { return m_countBits.hasRecordedMeasurement(feature); }
 
     CountBits m_countBits;
     BitVector m_CSSFeatureBits;
 };
 
 } // namespace blink
 
 #endif // UseCounter_h