No longer consider window size when scrolling

Now that scrolling roots are properly sized, we can simply look at the
scrolling root layer size when deciding the extents of layer scrolling.
This simplifies things a bit and further codifies the current model of
scrolling root + mask_to_bounds.
auto merge of #3544 : mrobinson/servo/simplify-scrolling, r=zwarich

Now that scrolling roots are properly sized, we can simply look at the
scrolling root layer size when deciding the extents of layer scrolling.
This simplifies things a bit and further codifies the current model of
scrolling root + mask_to_bounds.