Quote the getStyle method of the custom style element.

The method is referenced as a quoted property in ShadyCSS and so needs to be quoted here to ensure closure compiler ADVANCED mode property renaming does not rename this method.

lib/elements/custom-style.html

@@ -77,7 +77,7 @@
      *
      * @return {HTMLStyleElement} This element's light-DOM `<style>`
      */
-    getStyle() {
+    'getStyle'() {
       if (this._style) {
         return this._style;
       }