Add comments to tell UseCounter availability in branches.

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 221 matching lines @@
         DOMFocusInOutEvent = 211,
         FileGetLastModifiedDate = 212,
         HTMLElementInnerText = 213,
         HTMLElementOuterText = 214,
         ReplaceDocumentViaJavaScriptURL = 215,
         ElementPrefixedMatchesSelector = 217,
         DOMImplementationCreateCSSStyleSheet = 218,
         CSSStyleSheetRules = 219,
         CSSStyleSheetAddRule = 220,
         CSSStyleSheetRemoveRule = 221,
+        // The above items are available in M33 branch.
+
         InitMessageEvent = 222,
         PrefixedInitMessageEvent = 223,
         ElementSetPrefix = 224, // Element.prefix is readonly in DOM4.
         CSSStyleDeclarationGetPropertyCSSValue = 225,
         SVGElementGetPresentationAttribute = 226,
         PrefixedMediaCancelKeyRequest = 229,
         DOMImplementationHasFeature = 230,
         DOMImplementationHasFeatureReturnFalse = 231,
         CanPlayTypeKeySystem = 232,
         PrefixedDevicePixelRatioMediaFeature = 233,
@@ skipping 29 matching lines @@
         DeprecatedWebKitRepeatingRadialGradient = 264,
         PrefixedGetImageDataHD = 265,
         PrefixedPutImageDataHD = 266,
         PrefixedImageSmoothingEnabled = 267,
         UnprefixedImageSmoothingEnabled = 268,
         ShadowRootApplyAuthorStyles = 269,
         PromiseConstructor = 270,
         PromiseCast = 271,
         PromiseReject = 272,
         PromiseResolve = 273,
+        // The above items are available in M34 branch.
+
         TextAutosizing = 274,
         TextAutosizingLayout = 275,
         HTMLAnchorElementPingAttribute = 276,
         JavascriptExhaustedMemory = 277,
         InsertAdjacentHTML = 278,
         SVGClassName = 279,
         HTMLAppletElement = 280,
         HTMLMediaElementSeekToFragmentStart = 281,
         HTMLMediaElementPauseAtFragmentEnd = 282,
         PrefixedWindowURL = 283,
@@ skipping 81 matching lines @@
 
     void updateMeasurements();
 
     OwnPtr<BitVector> m_countBits;
     BitVector m_CSSFeatureBits;
 };
 
 } // namespace WebCore
 
 #endif // UseCounter_h