Move create() methods from StylePropertySet to MutableStylePropertySet.

Source/core/css/CSSComputedStyleDeclaration.cpp

Index: Source/core/css/CSSComputedStyleDeclaration.cpp
diff --git a/Source/core/css/CSSComputedStyleDeclaration.cpp b/Source/core/css/CSSComputedStyleDeclaration.cpp
index 1a3080a24ad99eba98cc68765deb367374449d2f..a5efe544f8b10af670faca9ebf0c146e325310a7 100644
--- a/Source/core/css/CSSComputedStyleDeclaration.cpp
+++ b/Source/core/css/CSSComputedStyleDeclaration.cpp
@@ -2905,7 +2905,7 @@ PassRefPtr<MutableStylePropertySet> CSSComputedStyleDeclaration::copyPropertiesI
         if (value)
             list.append(CSSProperty(properties[i], value.release(), false));
     }
-    return static_pointer_cast<MutableStylePropertySet>(StylePropertySet::create(list.data(), list.size()));
+    return MutableStylePropertySet::create(list.data(), list.size());
 }
 
 void CSSComputedStyleDeclaration::reportMemoryUsage(MemoryObjectInfo* memoryObjectInfo) const