LibWeb/CSS: Store the style sheet's source text on the CSSStyleSheet

This is to enable the inspector to show this source.

There's a fairly hefty FIXME here because duplicating the source text is
a significant waste of memory. But I don't want to get too sidetracked.

Userland/Libraries/LibWeb/CSS/CSSStyleSheet.cpp

@@ -371,4 +371,14 @@ void CSSStyleSheet::recalculate_namespaces()
     }
 }
 
+void CSSStyleSheet::set_source_text(String source)
+{
+    m_source_text = move(source);
+}
+
+Optional<String> CSSStyleSheet::source_text(Badge<DOM::Document>) const
+{
+    return m_source_text;
+}
+
 }