[Tizen][Linux] Support getting thumbnail for devtools.

 Using "ui::GrabViewSnapshotAsync" to implement interface
"XWalkDevToolsDelegate::GetPageThumbnailData".

BUG=https://crosswalk-project.org/jira/browse/XWALK-2883
One questions for this: How do you update thumbnail when the website changed? Maybe it is not a big deal, but I think you should put a TODO comment.