layout: Move overflow calculation to be a separate, sequential,

bottom-up pass.

Right now, the only reason that overflow calculation works is that we
rely on script inducing extra reflows that are sent for display. This
was preventing #10021 from landing.

This change regresses layout performance by about 1% in my tests.

Fixes #7797 properly.

components/layout/traversal.rs

@@ -210,7 +210,6 @@ impl<'a> PreorderFlowTraversal for ComputeAbsolutePositions<'a> {
     #[inline]
     fn process(&self, flow: &mut Flow) {
         flow.compute_absolute_position(self.layout_context);
-        flow.store_overflow(self.layout_context);
     }
 }