layout: Centralize the logic that determines whether fragments get

layers in the fragment, so that it can be activated when we're forcing
the creation of extra layers due to positioned descendants that
themselves have layers.

The newly failing tests were tests that accidentally passed due to
incorrect stacking order.

Closes #7281.

tests/wpt/metadata-css/css21_dev/html4/abspos-overflow-011.htm.ini

@@ -0,0 +1,3 @@
+[abspos-overflow-011.htm]
+  type: reftest
+  expected: FAIL