Move create() methods from StylePropertySet to MutableStylePropertySet.

Source/core/css/FontLoader.cpp

Index: Source/core/css/FontLoader.cpp
diff --git a/Source/core/css/FontLoader.cpp b/Source/core/css/FontLoader.cpp
index c731791dedc7bd94811e493fda6be5056f3a024f..564cfd8a924025a14d97bc98aa195dce6797914f 100644
--- a/Source/core/css/FontLoader.cpp
+++ b/Source/core/css/FontLoader.cpp
@@ -306,7 +306,7 @@ static void applyPropertyToCurrentStyle(StyleResolver* styleResolver, CSSPropert
 bool FontLoader::resolveFontStyle(const String& fontString, Font& font)
 {
     // Interpret fontString in the same way as the 'font' attribute of CanvasRenderingContext2D.
-    RefPtr<StylePropertySet> parsedStyle = StylePropertySet::create();
+    RefPtr<MutableStylePropertySet> parsedStyle = MutableStylePropertySet::create();
     CSSParser::parseValue(parsedStyle.get(), CSSPropertyFont, fontString, true, CSSStrictMode, 0);
     if (parsedStyle->isEmpty())
         return false;