Move create() methods from StylePropertySet to MutableStylePropertySet.

Source/core/editing/ApplyStyleCommand.cpp

Index: Source/core/editing/ApplyStyleCommand.cpp
diff --git a/Source/core/editing/ApplyStyleCommand.cpp b/Source/core/editing/ApplyStyleCommand.cpp
index b9daa3d9f6dd2a2586c684c7fc709d5db73f7c50..f4d8e7e28b50f996ef0752510211b3ef0af188e8 100644
--- a/Source/core/editing/ApplyStyleCommand.cpp
+++ b/Source/core/editing/ApplyStyleCommand.cpp
@@ -296,10 +296,10 @@ void ApplyStyleCommand::applyBlockStyle(EditingStyle *style)
         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();
 }