Add a comment about counter availability in M38 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 470 matching lines @@
         UIEventLayerX = 511,
         UIEventLayerY = 512,
         UIEventPageX = 513,
         UIEventPageY = 514,
         BgPropertiesFixed = 515,
         HTMLImageElementComposite = 516,
         DevToolsConsoleTimeline = 517,
         DevToolsConsoleProfile = 518,
         SVGStyleElementTitle = 519,
         PictureSourceSrc = 520,
+        // The above items are available in M38 branch.
+
         Picture = 521,
         Sizes = 522,
         SrcsetXDescriptor = 523,
         SrcsetWDescriptor = 524,
         SelectionContainsNode = 525,
         // 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 55 matching lines @@
 
     void updateMeasurements();
 
     OwnPtr<BitVector> m_countBits;
     BitVector m_CSSFeatureBits;
 };
 
 } // namespace blink
 
 #endif // UseCounter_h