Add a use counter of css clip of fixed-pos elements.

third_party/WebKit/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 1286 matching lines @@
         UsbDeviceSelectAlternateInterface = 1526,
         UsbDeviceControlTransferIn = 1527,
         UsbDeviceControlTransferOut = 1528,
         UsbDeviceClearHalt = 1529,
         UsbDeviceTransferIn = 1530,
         UsbDeviceTransferOut = 1531,
         UsbDeviceIsochronousTransferIn = 1532,
         UsbDeviceIsochronousTransferOut = 1533,
         UsbDeviceReset = 1534,
 
+        ClipCssOfFixedPositionElement = 1552,
         // 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);
     static void count(const Document&, Feature);
@@ skipping 69 matching lines @@
     friend class UseCounterTest;
     unsigned m_muteCount;
 
     CountBits m_countBits;
     BitVector m_CSSFeatureBits;
 };
 
 } // namespace blink
 
 #endif // UseCounter_h