Move create() methods from StylePropertySet to MutableStylePropertySet.

Source/core/editing/ApplyStyleCommand.cpp

 /*
  * Copyright (C) 2005, 2006, 2008, 2009 Apple 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 278 matching lines @@
         paragraphStart = nextParagraphStart;
         nextParagraphStart = endOfParagraph(paragraphStart).next();
     }
     
     startRange = TextIterator::rangeFromLocationAndLength(toContainerNode(scope), startIndex, 0, true);
     endRange = TextIterator::rangeFromLocationAndLength(toContainerNode(scope), endIndex, 0, true);
     if (startRange && endRange)
         updateStartEnd(startRange->startPosition(), endRange->startPosition());
 }
 
-static PassRefPtr<StylePropertySet> copyStyleOrCreateEmpty(const StylePropertySet* style)
+static PassRefPtr<MutableStylePropertySet> copyStyleOrCreateEmpty(const StylePropertySet* style)
 {
     if (!style)
-        return StylePropertySet::create();
+        return MutableStylePropertySet::create();
     return style->mutableCopy();
 }
 
 void ApplyStyleCommand::applyRelativeFontStyleChange(EditingStyle* style)
 {
     static const float MinimumFontSize = 0.1f;
 
     if (!style || !style->hasFontSizeDelta())
         return;
 
@@ skipping 1234 matching lines @@
         String textToMove = nextText->data();
         insertTextIntoNode(childText, childText->length(), textToMove);
         removeNode(next);
         // don't move child node pointer. it may want to merge with more text nodes.
     }
 
     updateStartEnd(newStart, newEnd);
 }
 
 }