wl_surface: clarify the base of time passed in the callback of frame
wl_surface: clarify the base of time passed in the callback of frame

Signed-off-by: Ryo Munakata <b0e96e5000c47678832fb87f79b32c3d1828548c@gmail.com>
Reviewed-by: Pekka Paalanen <0f6ae7001e34c37b02fb2c1c2573ddf8f442136a@collabora.co.uk>

abooij/wayland,bitfactor/wayland,tulcod/wayland,abooij/wayland,bitfactor/wayland,sir-murray/wayland,sir-murray/wayland,tulcod/wayland,tulcod/wayland,mchalupa/wayland,eyolfson/wayland,eyolfson/wayland,mchalupa/wayland,mchalupa/wayland,abooij/wayland,eyolfson/wayland,bitfactor/wayland,sir-murray/wayland