If grid-template-rows or grid-template-columns queried for a box that is
not a grid container, the result should be computed value instead of
null.
Fixes crashing in inspector.
That's awkward, but getComputedStyle needs to return used track values
for gridTemplateColumns and gridTemplateRows properties. This change
implements it by saving style values with used values into layout state,
so it could be assigned to paintables during LayoutState::commit() and
later accessed by style_value_for_property().
I haven't seen it used in the wild, but WPT grid tests extensively use
it. For example this change helps to go from 0/10 to 8/10 on this test:
https://wpt.live/css/css-grid/layout-algorithm/grid-fit-content-percentage.html
