Update to use latest XWalkView API

Work with xwalk_core_library version 6.35.124.0
Use composite instead of inherit to implement XWalkCordovaWebView.

Inheriting XWalkView causes issues when calling some APIs.
For example, canGoBack() is declared in XWalkView internally. If
XWalkCordovaWebView implements its canGoBack(), the XWalkView internal code
will call it.

Generally, to implement the CordovaWebView interface and delegate to WebView
implementations seems a good approach.
Merge pull request #3 from huningxin/xwalk-api-update

Xwalk embedding api update