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

Source/platform/graphics/GraphicsTypes.h

 /*
  * Copyright (C) 2004, 2005, 2006 Apple Computer, 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 49 matching lines @@
     CompositeCopy,
     CompositeSourceOver,
     CompositeSourceIn,
     CompositeSourceOut,
     CompositeSourceAtop,
     CompositeDestinationOver,
     CompositeDestinationIn,
     CompositeDestinationOut,
     CompositeDestinationAtop,
     CompositeXOR,
-    CompositePlusDarker,
-    CompositePlusLighter,
-    CompositeDifference
+    CompositePlusLighter
 };
 
 enum ShadowMode {
     DrawShadowAndForeground,
     DrawShadowOnly,
 };
 
 enum AntiAliasingMode {
     NotAntiAliased,
     AntiAliased
@@ skipping 52 matching lines @@
 
 PLATFORM_EXPORT String textAlignName(TextAlign);
 PLATFORM_EXPORT bool parseTextAlign(const String&, TextAlign&);
 
 PLATFORM_EXPORT String textBaselineName(TextBaseline);
 PLATFORM_EXPORT bool parseTextBaseline(const String&, TextBaseline&);
 
 } // namespace blink
 
 #endif