Move CSSOM classes to using MutableStylePropertySet over StylePropertySet.

Source/core/editing/FrameSelection.h

Index: Source/core/editing/FrameSelection.h
diff --git a/Source/core/editing/FrameSelection.h b/Source/core/editing/FrameSelection.h
index 429c689401eeeb94dcc88fdd15f64ff4bb39d4af..119d04c07fb9170f75d60636c9e437b6dfb901c5 100644
--- a/Source/core/editing/FrameSelection.h
+++ b/Source/core/editing/FrameSelection.h
@@ -41,10 +41,10 @@ class CharacterData;
 class Frame;
 class GraphicsContext;
 class HTMLFormElement;
+class MutableStylePropertySet;
 class RenderObject;
 class RenderView;
 class Settings;
-class StylePropertySet;
 class VisiblePosition;
 
 enum EUserTriggered { NotUserTriggered = 0, UserTriggered = 1 };
@@ -243,7 +243,7 @@ public:
     void paintDragCaret(GraphicsContext*, const LayoutPoint&, const LayoutRect& clipRect) const;
 
     EditingStyle* typingStyle() const;
-    PassRefPtr<StylePropertySet> copyTypingStyle() const;
+    PassRefPtr<MutableStylePropertySet> copyTypingStyle() const;
     void setTypingStyle(PassRefPtr<EditingStyle>);
     void clearTypingStyle();