Implement page.$$ method (#463)

This patch implements page.$$ which runs document.querySelectorAll
in page and returns results as an array of ElementHandle instances.

Fixes #384.

lib/FrameManager.js

@@ -191,10 +191,33 @@ class Frame {
     const remoteObject = await this._rawEvaluate(selector => document.querySelector(selector), selector);
     if (remoteObject.subtype === 'node')
       return new ElementHandle(this._client, remoteObject, this._mouse);
-    helper.releaseObject(this._client, remoteObject);
+    await helper.releaseObject(this._client, remoteObject);
     return null;
   }
 
+  /**
+   * @param {string} selector
+   * @return {!Promise<!Array<!ElementHandle>>}
+   */
+  async $$(selector) {
+    const remoteObject = await this._rawEvaluate(selector => Array.from(document.querySelectorAll(selector)), selector);
+    const response = await this._client.send('Runtime.getProperties', {
+      objectId: remoteObject.objectId,
+      ownProperties: true
+    });
+    const properties = response.result;
+    const result = [];
+    const releasePromises = [helper.releaseObject(this._client, remoteObject)];
+    for (const property of properties) {
+      if (property.enumerable && property.value.subtype === 'node')
+        result.push(new ElementHandle(this._client, property.value, this._mouse));
+      else
+        releasePromises.push(helper.releaseObject(this._client, property.value));
+    }
+    await Promise.all(releasePromises);
+    return result;
+  }
+
   /**
    * @param {function()|string} pageFunction
    * @param {!Array<*>} args