LibWeb: Update existing style object when setting style attribute

Previously any existing ElementInlineCSSStyleDeclaration would get
overwritten by e.setAttribute("style", ...), while it should be updated
instead.

This fixes 2 WPT subtests.

Libraries/LibWeb/CSS/CSSStyleDeclaration.h

@@ -123,6 +123,8 @@ public:
 
     bool is_updating() const { return m_updating; }
 
+    void set_declarations_from_text(StringView);
+
     virtual WebIDL::ExceptionOr<void> set_css_text(StringView) override;
 
 private: