Move CSSOM classes to using MutableStylePropertySet over StylePropertySet.

Source/core/inspector/InspectorCSSAgent.cpp

Index: Source/core/inspector/InspectorCSSAgent.cpp
diff --git a/Source/core/inspector/InspectorCSSAgent.cpp b/Source/core/inspector/InspectorCSSAgent.cpp
index 49e9b0783211fec59dd790288d640ba8fdc8dd2e..5102d367cba41bf038ae7dd535ab91b791f56b79 100644
--- a/Source/core/inspector/InspectorCSSAgent.cpp
+++ b/Source/core/inspector/InspectorCSSAgent.cpp
@@ -1455,11 +1455,15 @@ PassRefPtr<TypeBuilder::CSS::CSSStyle> InspectorCSSAgent::buildObjectForAttribut
     if (!element->isStyledElement())
         return 0;
 
-    const StylePropertySet* attributeStyle = static_cast<StyledElement*>(element)->presentationAttributeStyle();
+    // FIXME: Ugliness below.
+    StylePropertySet* attributeStyle = const_cast<StylePropertySet*>(static_cast<StyledElement*>(element)->presentationAttributeStyle());
     if (!attributeStyle)
         return 0;
 
-    RefPtr<InspectorStyle> inspectorStyle = InspectorStyle::create(InspectorCSSId(), const_cast<StylePropertySet*>(attributeStyle)->ensureCSSStyleDeclaration(), 0);
+    ASSERT(attributeStyle->isMutable());
+    MutableStylePropertySet* mutableAttributeStyle = static_cast<MutableStylePropertySet*>(attributeStyle);
+
+    RefPtr<InspectorStyle> inspectorStyle = InspectorStyle::create(InspectorCSSId(), mutableAttributeStyle->ensureCSSStyleDeclaration(), 0);
     return inspectorStyle->buildObjectForStyle();
 }