fx-compat: Initial fix for Scaffold

Not working:
- Web tests (hidden browser is broken)
- Test Updated status text
- Various platform menu things (code copied from ZP and will use whichever
  approach we settle on there)

chrome/content/scaffold/content.js

@@ -0,0 +1,15 @@
+addMessageListener('Scaffold:GetDocument', {
+	receiveMessage() {
+		dump('received document request\n')
+		sendAsyncMessage('Scaffold:Document', {
+			html: new XMLSerializer().serializeToString(content.document),
+			url: content.location.href
+		});
+	}
+});
+
+function onLoad() {
+	sendAsyncMessage('Scaffold:Load', { url: content.location.href });
+}
+
+addEventListener('load', onLoad, true);