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

 Using "ui::GrabViewSnapshotAsync" to implement interface
"XWalkDevToolsDelegate::GetPageThumbnailData".

BUG=https://crosswalk-project.org/jira/browse/XWALK-2883
I agree the out-of-date snapshot is kind of okay, so please leaving TODO is enough for me.