Avoid using string constructor of QJSValue for byte-array

We were returning the PDF byte-data as a javascript unicode string using
the deprecated ascii cast constructor.

This patch changes the behavior to match that of runJavaScript callbacks
and uses a script value over QVariant.

[ChangeLog][QtWebEngine] The callback version of WebEngineView::printToPdf
is now called with a proper bytearray result instead of PDF byte data
in a javascript string.

Change-Id: I71565623465c54052568bb5ff34665baaa93e187
Reviewed-by: Michal Klocek <michal.klocek@qt.io>