If any of you remember Earth Science in H.S. and Mohs hardness scale, glass
falls at about 5.5 and finger nails(including dogs claws) comes in about 3.
 In other words the dogs nails would be scratched by the glass and not the
other way around.  The stuff should come off without much trouble.  The
only other thing that could have happened is the dogs may have tracked some
sand, ie quartz grains, onto the windshield.  This would easily scratch the
glass and in that case what do you do?

Dave K.
Certified rock nerd, uh I mean geologist