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

Source/core/css/CSSPrimitiveValueMappings.h

 /*
  * Copyright (C) 2007 Alexey Proskuryakov <ap@nypop.com>.
  * Copyright (C) 2008, 2009, 2010, 2011 Apple Inc. All rights reserved.
  * Copyright (C) 2009 Torch Mobile Inc. All rights reserved. (http://www.torchmobile.com/)
  * Copyright (C) 2009 Jeff Schiller <codedread@gmail.com>
  * Copyright (C) Research In Motion Limited 2010. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
  * are met:
@@ skipping 330 matching lines @@
         break;
     case CompositeDestinationOut:
         m_value.valueID = CSSValueDestinationOut;
         break;
     case CompositeDestinationAtop:
         m_value.valueID = CSSValueDestinationAtop;
         break;
     case CompositeXOR:
         m_value.valueID = CSSValueXor;
         break;
-    case CompositePlusDarker:
-        m_value.valueID = CSSValuePlusDarker;
-        break;
     case CompositePlusLighter:
         m_value.valueID = CSSValuePlusLighter;
         break;
-    case CompositeDifference:
+    default:
         ASSERT_NOT_REACHED();
         break;
     }
 }
 
 template<> inline CSSPrimitiveValue::operator CompositeOperator() const
 {
     ASSERT(isValueID());
     switch (m_value.valueID) {
     case CSSValueClear:
@@ skipping 11 matching lines @@
     case CSSValueDestinationOver:
         return CompositeDestinationOver;
     case CSSValueDestinationIn:
         return CompositeDestinationIn;
     case CSSValueDestinationOut:
         return CompositeDestinationOut;
     case CSSValueDestinationAtop:
         return CompositeDestinationAtop;
     case CSSValueXor:
         return CompositeXOR;
-    case CSSValuePlusDarker:
-        return CompositePlusDarker;
     case CSSValuePlusLighter:
         return CompositePlusLighter;
     default:
         break;
     }
 
     ASSERT_NOT_REACHED();
     return CompositeClear;
 }
 
@@ skipping 4377 matching lines @@
     default:
         break;
     }
     ASSERT_NOT_REACHED();
     return ScrollBehaviorAuto;
 }
 
 }
 
 #endif