Fix that CordovaChromeClient misses all the implementation in xwalk

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

BUG=https://crosswalk-project.org/jira/browse/XWALK-954

It's cherry-picked from 073e1b1f.
Exit fullscreen mode first when back key pressed

In fullscreen mode, back key is used to exit fullscreen
prior to navigation.
The overall back key handling priority is:
1. Exit fullscreen
2. Exit video
3. Let JS handle
4. Go backward
5. Finish activity.

It's cherry-picked from 7bdede21.
Merge pull request #95 from wang16/xwalk-4

Fix fullscreen issue on beta branch