Add a comment about counter availability in M44 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 669 matching lines @@
         AutocapitalizeAttribute = 764,
         FullscreenSecureOrigin = 765,
         FullscreenInsecureOrigin = 766,
         DialogInSandboxedContext = 767,
         SVGSMILAnimationInImageRegardlessOfCache = 768,
         PushSubscriptionId = 769,
         EncryptedMediaSecureOrigin = 770,
         EncryptedMediaInsecureOrigin = 771,
         PerformanceFrameTiming = 772,
         V8Element_Animate_Method = 773,
+        // The above items are available in M44 branch.
+
         V8SVGSVGElement_PixelUnitToMillimeterX_AttributeGetter = 774,
         V8SVGSVGElement_PixelUnitToMillimeterY_AttributeGetter = 775,
         V8SVGSVGElement_ScreenPixelToMillimeterX_AttributeGetter = 776,
         V8SVGSVGElement_ScreenPixelToMillimeterY_AttributeGetter = 777,
         V8SVGSVGElement_GetElementById_Method = 778,
 
         // 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.
@@ skipping 88 matching lines @@
 
     bool hasRecordedMeasurement(Feature feature) const { return m_countBits.hasRecordedMeasurement(feature); }
 
     CountBits m_countBits;
     BitVector m_CSSFeatureBits;
 };
 
 } // namespace blink
 
 #endif // UseCounter_h