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

 Using "ui::GrabViewSnapshotAsync" to implement interface
"XWalkDevToolsDelegate::GetPageThumbnailData".

BUG=https://crosswalk-project.org/jira/browse/XWALK-2883
Actually I was considering that for a while. My thought was:
(1) The main purpose of thumbnail in front-end is to help user to recognize app, so first snapshot might be enough.
(2) Actually, I can move the "if(...) return thumbnail;" to the end of "GetPageThumbnailData" so we can get new snapshot every time, since the design of the devtools will invoke "GetPageThumbnailData" two times to get one thumbnail (one time to get target list, one time to get corespondent thumbnail). If the real-time snapshot is really important, we take this way.
(3) Actually, we can make it more straightforward that just invoke the sync function "ui::GrabViewSnapshot", however, it's unsupported for Aura :(.