TBR: devtools/vkiosk: use scrot to take screenshot which works better with Xvfb.
TBR: devtools/vkiosk: use scrot to take screenshot which works better with Xvfb.

PresubmitTest: none
Change-Id: Id629dd6454f8d1cb5da799e1f3eacfbc2dacd15b

vanadium/go.devtools,vanadium/go.devtools,vanadium/go.devtools