[Android] Fix XWALK unable to support download behavior

BUG=https://crosswalk-project.org/jira/browse/XWALK-1079
@wang16 
https://code.google.com/p/chromium/issues/detail?id=432414&thanks=432414&ts=1415773645
This is the bug link, seems no response yet.

There are 2 implementions from upstream, while Crosswalk's implementation is almost the same with Androidwebview for the part which support download. AndroidWebview does not support this download too...The other implementation is from Chrome Shell, which is very different from XWalk's