Resolved grid-template-columns and grid-template-rows values are needed
for getComputedStyle(), even when DevTools is not connected. Store them
separately from GridLayoutData again so the later lazy DevTools data
work can skip inspector-only geometry without changing CSSOM behavior.
Partly reverts 540b53ac1a.
Store grid container layout data on the grid container's paintable box
during layout.
The data includes computed track, line, and named-area geometry for
Firefox DevTools protocol serialization. It also carries the resolved
grid-template-columns and grid-template-rows values used by
getComputedStyle(), so PaintableBox does not need separate grid
template storage.