feat(JSHandles): introduce JSHandles (#943)

This patch:
- introduces ExecutionContext class that incapsulates javascript
  execution context. An examples of execution contexts are workers and
  frames
- introduces JSHandle that holds a references to the javascript
  object in ExecutionContext
- inherits ElementHandle from JSHandle

Fixes #382.

lib/helper.js

@@ -64,7 +64,8 @@ class Helper {
    * @param {!Object} remoteObject
    * @return {!Promise<!Object>}
    */
-  static async serializeRemoteObject(client, remoteObject) {
+  static valueFromRemoteObject(remoteObject) {
+    console.assert(!remoteObject.objectId, 'Cannot extract value when objectId is given');
     if (remoteObject.unserializableValue) {
       switch (remoteObject.unserializableValue) {
         case '-0':
@@ -79,8 +80,17 @@ class Helper {
           throw new Error('Unsupported unserializable value: ' + remoteObject.unserializableValue);
       }
     }
+    return remoteObject.value;
+  }
+
+  /**
+   * @param {!Session} client
+   * @param {!Object} remoteObject
+   * @return {!Promise<!Object>}
+   */
+  static async serializeRemoteObject(client, remoteObject) {
     if (!remoteObject.objectId)
-      return remoteObject.value;
+      return Helper.valueFromRemoteObject(remoteObject);
     if (remoteObject.subtype === 'promise')
       return remoteObject.description;
     try {