mw.loader: Use requestAnimationFrame for addEmbeddedCSS()

mw.loader: Use requestAnimationFrame for addEmbeddedCSS()

setTimeout is fairly inefficient for this purpose as it tends to schedule for
further in the future than rAF would. And even then, it happens at a bad time
for the browser with regards to style changes.

Instead, use rAF, which typically executes earlier and at the point where the
browser is expecting style changes.

This makes top and bottom modules finish execution earlier by having their
styles applied sooner.

Change-Id: Ie4e7464aa811fa8ea4e4f115696f0bddbd28737b