`grid-template-rows: fit-content(100px)` had no effect when the grid
container lacked an explicit height, because the fit-content growth
limit clamping was gated behind `available_size.is_definite()`.
Fix by normalizing fit-content tracks with unresolvable percentage
arguments to max-content during track initialization, then applying
the fit-content clamp unconditionally.
Fixes https://github.com/LadybirdBrowser/ladybird/issues/7730
