Add a comment about counter availability in M40 branch

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 539 matching lines @@
         CSSSelectorPseudoScrollbarButton = 583,
         CSSSelectorPseudoScrollbarThumb = 584,
         CSSSelectorPseudoScrollbarTrack = 585,
         CSSSelectorPseudoScrollbarTrackPiece = 586,
         LangAttribute = 587,
         LangAttributeOnHTML = 588,
         LangAttributeOnBody = 589,
         LangAttributeDoesNotMatchToUILocale = 590,
         InputTypeSubmit = 591,
         InputTypeSubmitWithValue = 592,
+        // The above items are available in M40 branch.
+
         SetReferrerPolicy = 593,
         DOMImplementationHasFeatureReturnFalseInternal = 594,
 
         // 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.
     };
 
@@ skipping 65 matching lines @@
 
     void updateMeasurements();
 
     OwnPtr<BitVector> m_countBits;
     BitVector m_CSSFeatureBits;
 };
 
 } // namespace blink
 
 #endif // UseCounter_h