Scroll forward LINES lines in View mode, setting the "page size".
This is the number of lines which M-x View-scroll-page-forward and
M-x View-scroll-page-backward scroll by default.
If LINES is omitted or = 0, sets "page size" to window height and
scrolls forward that much, otherwise scrolls forward LINES lines and sets
"page size" to the minimum of window height and the absolute value of LINES.
See also `View-scroll-page-forward'.