[Android] Fix crash issue when destruct XWalkView

It's caused by destruction order of WebContents and WebContentsDelegate.

The WebContentsDelegate need to take ownership of WebContents
as chrome content design. For xwalk, XwalkContent owns these two,
Currently as a workaround, reorder the declaration to make sure the
WebContents destructed before WebContentsDelegate.

BUG=XWALK-1639