[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

(Cherry picked from commit f0cd0a3614c7a0e872e80e36113f08838178c421)
Merge pull request #1946 from kurli/cherry-pick-fix-exit-crash

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