feat(Frame): Add addStyleTag API to frame - fixes #892 (#947)

This patch adds `Page.addStyleTag` and `Frame.addStyleTag` methods to the API.

Fixes #892.

lib/FrameManager.js

@@ -358,6 +358,25 @@ class Frame {
     }
   }
 
+  /**
+   * @param {string} url
+   */
+  async addStyleTag(url) {
+    return this.evaluate(addStyleTag, url);
+
+    /**
+     * @param {string} url
+     */
+    function addStyleTag(url) {
+      const link = document.createElement('link');
+      link.rel = 'stylesheet';
+      link.href = url;
+      const promise = new Promise(x => link.onload = x);
+      document.head.appendChild(link);
+      return promise;
+    }
+  }
+
   /**
    * @param {(string|number|function())} selectorOrTimeout
    * @param {!Object=} options