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

Using "ui::GrabViewSnapshotAsync" to implement interface
"XWalkDevToolsDelegate::GetPageThumbnailData".

BUG=https://crosswalk-project.org/jira/browse/XWALK-2883
Merge pull request #2571 from iKevinHan/implement_devtools_thumb

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