LibHTML: Make StyleResolver responsible for loading the default style

Instead of HtmlView clients having to worry about parsing and loading
the default CSS, just take care of it inside StyleResolver.

The default style is automatically inserted into the stylesheet list,
at the very start, so everyone else gets a chance to override it.

Libraries/LibHTML/CSS/StyleResolver.h

@@ -23,5 +23,8 @@ public:
     NonnullRefPtrVector<StyleRule> collect_matching_rules(const Element&) const;
 
 private:
+    template<typename Callback>
+    void for_each_stylesheet(Callback) const;
+
     Document& m_document;
 };