Add a comment about counter availability in M45 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 765 matching lines @@
         V8Console_Memory_AttributeGetter = 877,
         V8Console_Memory_AttributeSetter = 878,
         V8MemoryInfo_TotalJSHeapSize_AttributeGetter = 879,
         V8MemoryInfo_UsedJSHeapSize_AttributeGetter = 880,
         V8MemoryInfo_JSHeapSizeLimit_AttributeGetter = 881,
         V8Performance_Timing_AttributeGetter = 882,
         V8Performance_Navigation_AttributeGetter = 883,
         V8Performance_Memory_AttributeGetter = 884,
         V8SharedWorker_WorkerStart_AttributeGetter = 885,
         HTMLKeygenElement = 886,
+        // The above items are available in M45 branch.
+
         V8SVGElement_OffsetParent_AttributeGetter = 887,
         V8SVGElement_OffsetTop_AttributeGetter = 888,
         V8SVGElement_OffsetLeft_AttributeGetter = 889,
         V8SVGElement_OffsetWidth_AttributeGetter = 890,
         V8SVGElement_OffsetHeight_AttributeGetter = 891,
 
         // 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