Comply Compositing spec [1]; remove 'plus-darker'.

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 495 matching lines @@
         SVG1DOM = 567,
         SVGPathSegDOM = 568,
         SVGTransformListConsolidate = 569,
         SVGAnimatedTransformListBaseVal = 570,
         QuotedAnimationName = 571,
         QuotedKeyframesRule = 572,
         SrcsetDroppedCandidate = 573,
         WindowPostMessage = 574,
         WindowPostMessageWithLegacyTargetOriginArgument = 575,
         RenderRuby = 576,
-        CanvasRenderingContext2DCompositeOperationDarker = 577,
         ScriptElementWithInvalidTypeHasSrc = 578,
         TimelineStart = 579,
         ElementBaseURIFromXMLBase = 580,
         XMLHttpRequestSynchronousInNonWorkerOutsideBeforeUnload = 581,
         CSSSelectorPseudoScrollbar = 582,
         CSSSelectorPseudoScrollbarButton = 583,
         CSSSelectorPseudoScrollbarThumb = 584,
         CSSSelectorPseudoScrollbarTrack = 585,
         CSSSelectorPseudoScrollbarTrackPiece = 586,
         LangAttribute = 587,
@@ skipping 161 matching lines @@
 
     void updateMeasurements();
 
     OwnPtr<BitVector> m_countBits;
     BitVector m_CSSFeatureBits;
 };
 
 } // namespace blink
 
 #endif // UseCounter_h