[Android] Reordered the declaration of WebContents and WebContentsDelegate in XWalkContent

It will cause crash when destroy XWalkContent, caused by destruction order.

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

BUG=XWALK-1639
Merge pull request #1942 from kurli/fix-exit-crash

[Android] Reordered the declaration of WebContents and WebContentsDelegate in XWalkContent