Add a comment about counter availability in M41 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 587 matching lines @@
         RemoveEventListenerNoArguments = 658,
         RemoveEventListenerOneArgument = 659,
         SRIElementWithNonMatchingIntegrityType = 660,
         MixedContentInNonHTTPSFrameThatRestrictsMixedContent = 661,
         MixedContentInSecureFrameThatDoesNotRestrictMixedContent = 662,
         MixedContentWebSocket = 663,
         SyntheticKeyframesInCompositedCSSAnimation = 664,
         MixedContentFormPresent = 665,
         GetUserMediaInsecureOrigin = 666,
         GetUserMediaSecureOrigin = 667,
+        // The above items are available in M41 branch.
+
         DeviceMotionInsecureOrigin = 668,
         DeviceMotionSecureOrigin = 669,
         DeviceOrientationInsecureOrigin = 670,
         DeviceOrientationSecureOrigin = 671,
         SandboxViaIFrame = 672,
         SandboxViaCSP = 673,
         BlockedSniffingImageToScript = 674,
         Fetch = 675,
         FetchBodyStream = 676,
 
@@ skipping 72 matching lines @@
 
     void updateMeasurements();
 
     OwnPtr<BitVector> m_countBits;
     BitVector m_CSSFeatureBits;
 };
 
 } // namespace blink
 
 #endif // UseCounter_h