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.

tests/wpt/metadata-css/css21_dev/html4/numbers-units-018.htm.ini

@@ -0,0 +1,3 @@
+[numbers-units-018.htm]
+  type: reftest
+  expected: FAIL