[api] Introduce Page.select method (#779)

This patch adds `Page.select` method to select
values in a `select` tag.

lib/Page.js

@@ -693,6 +693,29 @@ class Page extends EventEmitter {
     await handle.dispose();
   }
 
+  /**
+   * @param {string} selector
+   * @param {!Array<string>} values
+   */
+  async select(selector, ...values) {
+    await this.$eval(selector, (element, values) => {
+      if (element.nodeName.toLowerCase() !== 'select')
+        throw new Error('Element is not a <select> element.');
+
+      const options = Array.from(element.options);
+
+      if (element.multiple) {
+        for (const option of options)
+          option.selected = values.includes(option.value);
+      } else {
+        element.value = values.shift();
+      }
+
+      element.dispatchEvent(new Event('change'));
+      element.dispatchEvent(new Event('input'));
+    }, values);
+  }
+
   /**
    * @param {string} text
    * @param {{delay: (number|undefined)}=} options