ensure this.chrome is defined

lib/browser-polyfill/chrome-browser-polyfill.js

@@ -26,7 +26,7 @@
 
 	const FEATURE_TESTS = {};
 
-	if (isChrome && !this.browser) {
+	if (isChrome && !this.browser && this.chrome) {
 		const nativeAPI = this.chrome;
 		this.__defineGetter__("browser", () => ({
 			browserAction: {