Add a comment about counter availability in M43 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 664 matching lines @@
         UpgradeInsecureRequestsUpgradedRequest = 741,
         DocumentDesignMode = 742,
         GlobalCacheStorage = 743,
         NetInfo = 744,
         BackgroundSync = 745,
         TabStopProperty = 746,
         TabStopAttribute = 747,
         LegacyConst = 748,
         SVGFilterRes = 749,
         V8Permissions_Query_Method = 750,
+        // The above items are available in M43 branch.
+
         LegacyCSSValueIntrinsic = 751,
         LegacyCSSValueMinIntrinsic = 752,
         WebkitCanvas = 753,
         V8HTMLInputElement_Autocapitalize_AttributeGetter = 754,
         V8HTMLInputElement_Autocapitalize_AttributeSetter = 755,
         V8HTMLTextAreaElement_Autocapitalize_AttributeGetter = 756,
         V8HTMLTextAreaElement_Autocapitalize_AttributeSetter = 757,
 
         // Add new features immediately above this line. Don't change assigned
         // numbers of any item, and don't reuse removed slots.
@@ skipping 90 matching lines @@
 
     bool hasRecordedMeasurement(Feature feature) const { return m_countBits.hasRecordedMeasurement(feature); }
 
     CountBits m_countBits;
     BitVector m_CSSFeatureBits;
 };
 
 } // namespace blink
 
 #endif // UseCounter_h