Removed -webkit-app-region: drag from printer sample.

"-webkit-app-region: drag" is not compatible with the print preview
dialog (/crbug.com/309783), so this sample was changed to avoid using
it. This involved giving windows "frame: 'chrome'", which required some
workarounds for /crbug.com/309966, /crbug.com/309969, /crbug.com/309971,
and /crbug.com/310289.

The "Print current window" was also changed so it's disabled when there
are no windows to print.