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

Source/platform/graphics/GraphicsTypes.cpp

 /*
  * Copyright (C) 2006 Apple Computer, Inc.  All rights reserved.
  * Copyright (C) 2012 Rik Cabanier (cabanier@adobe.com)
  *
  * 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
@@ skipping 26 matching lines @@
     "copy",
     "source-over",
     "source-in",
     "source-out",
     "source-atop",
     "destination-over",
     "destination-in",
     "destination-out",
     "destination-atop",
     "xor",
-    "darker",
     "lighter"
 };
 
 static const char* const blendOperatorNames[] = {
     "normal",
     "multiply",
     "screen",
     "overlay",
     "darken",
     "lighten",
@@ skipping 156 matching lines @@
         return true;
     }
     if (s == "hanging") {
         baseline = HangingTextBaseline;
         return true;
     }
     return false;
 }
 
 } // namespace blink