Fix percentage height calculation, absolute containing block height calculations.

It's not possible to correctly determine during the css cascade whether the container height
is explicitly specified. Additionally, the spec https://drafts.csswg.org/css2/visudet.html#the-height-property
says this should affect the *used* height, rather than the computed height.

This significantly improves the layout in #6643.

tests/wpt/metadata-css/css21_dev/html4/block-in-inline-percents-001.htm.ini

@@ -1,3 +0,0 @@
-[block-in-inline-percents-001.htm]
-  type: reftest
-  expected: FAIL