LibWeb: Keep grid template used values outside DevTools data

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 540b53ac1a78dacbec0ac39bb9de1159c3d24a47.