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/profile_traits/time.rs

@@ -50,6 +50,7 @@ pub enum ProfilerCategory {
     LayoutGeneratedContent,
     LayoutDisplayListSorting,
     LayoutMain,
+    LayoutStoreOverflow,
     LayoutParallelWarmup,
     LayoutDispListBuild,
     PaintingPerTile,