Add UseCounter for FontFaceSet#ready()

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 443 matching lines @@
         WebkitEditableContentChangedOnNotNode = 454,
         HTMLImports = 455,
         ElementCreateShadowRoot = 456,
         DocumentRegisterElement = 457,
         EditingAppleInterchangeNewline = 458,
         EditingAppleConvertedSpace = 459,
         EditingApplePasteAsQuotation = 460,
         EditingAppleStyleSpanClass = 461,
         EditingAppleTabSpanClass = 462,
         HTMLImportsAsyncAttribute = 463,
+        FontFaceSetReady = 464,
         // 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 Document&, Feature);
     // This doesn't count for non-Document ExecutionContext.
@@ skipping 45 matching lines @@
 
     void updateMeasurements();
 
     OwnPtr<BitVector> m_countBits;
     BitVector m_CSSFeatureBits;
 };
 
 } // namespace WebCore
 
 #endif // UseCounter_h