Fix that CordovaChromeClient misses all the implementation in xwalk

The implementation in xwalk is in XWalkDefaultChromeClient, however
CordovaChromeClient extends XWalkChromeClient which barely has
real implementation. That causes many features missing in cordova
container.

BUG=https://crosswalk-project.org/jira/browse/XWALK-954
Align with xwalk to handle key up event

In fullscreen mode, back key is used to exit fullscreen
prior to navigation.