Move CSSOM classes to using MutableStylePropertySet over StylePropertySet.

Source/core/dom/Attr.h

Index: Source/core/dom/Attr.h
diff --git a/Source/core/dom/Attr.h b/Source/core/dom/Attr.h
index 85fc2c7121f37f342e3cdeb63b99d28853a7b6eb..5bc6de228c726291dcf1551cd998ca4f96c0c800 100644
--- a/Source/core/dom/Attr.h
+++ b/Source/core/dom/Attr.h
@@ -31,7 +31,7 @@
 namespace WebCore {
 
 class CSSStyleDeclaration;
-class StylePropertySet;
+class MutableStylePropertySet;
 
 // Attr can have Text children
 // therefore it has to be a fullblown Node. The plan
@@ -96,7 +96,7 @@ private:
     QualifiedName m_name;
     AtomicString m_standaloneValue;
 
-    RefPtr<StylePropertySet> m_style;
+    RefPtr<MutableStylePropertySet> m_style;
     unsigned m_ignoreChildrenChanged : 31;
     bool m_specified : 1;
 };