Add comments about counter availability in M46 and M47 branches.

third_party/WebKit/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 795 matching lines @@
         PresentationRequestReconnect = 930,
         PresentationRequestGetAvailability = 931,
         PresentationRequestConnectionAvailableEventListener = 932,
         PresentationConnectionClose = 933,
         PresentationConnectionSend = 934,
         PresentationConnectionStateChangeEventListener = 935,
         PresentationConnectionMessageEventListener = 936,
         CSSAnimationsStackedNeutralKeyframe = 937,
         ReadingCheckedInClickHandler = 938,
         FlexboxIntrinsicSizeAlgorithmIsDifferent = 939,
+        // The above items are available in M46 branch.
+
         HTMLImportsHasStyleSheets = 940,
         WebkitTextInClipProperty = 941,
         WebkitTextInColorProperty = 942,
         HeaderValueNotMatchingRFC7230 = 943,
         ClipPathOfPositionedElement = 944,
         ClipCssOfPositionedElement = 945,
         NetInfoType = 946,
         NetInfoDownlinkMax = 947,
         NetInfoOnChange = 948,
         NetInfoOnTypeChange = 949,
         V8Window_Alert_Method = 950,
         V8Window_Confirm_Method = 951,
         V8Window_Prompt_Method = 952,
         V8Window_Print_Method = 953,
         V8Window_RequestIdleCallback_Method = 954,
         FlexboxPercentagePaddingVertical = 955,
         FlexboxPercentageMarginVertical = 956,
         BackspaceNavigatedBack = 957,
         BackspaceNavigatedBackAfterFormInteraction = 958,
         CSPSourceWildcardWouldMatchExactHost = 959,
         CredentialManagerGet = 960,
         CredentialManagerGetWithUI = 961,
         CredentialManagerGetWithoutUI = 962,
         CredentialManagerStore = 963,
         CredentialManagerRequireUserMediation = 964,
+        // The above items are available in M47 branch.
+
         RequestAutocomplete = 965,
 
         // 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.
     };
 
     // "count" sets the bit for this feature to 1. Repeated calls are ignored.
@@ skipping 85 matching lines @@
 
     bool hasRecordedMeasurement(Feature feature) const { return m_countBits.hasRecordedMeasurement(feature); }
 
     CountBits m_countBits;
     BitVector m_CSSFeatureBits;
 };
 
 } // namespace blink
 
 #endif // UseCounter_h