Update layer bounds on resize

This fixes a bug where fixed-position layers are not repositioned when the
window is resized.
auto merge of #4412 : mbrubeck/servo/fixed-layer-resize, r=mrobinson

This fixes a bug where fixed-position layers are not repositioned when the
window is resized.  This can be reproduced with any `position: fixed` element with a `right` or `bottom` position.  I'm not sure how to reftest this, though.

r? @mrobinson