[css-grid] Fix placement for unknown named grid lines
The spec has changed and now all the implicit lines should be considered
when we're resolving named grid lines with an unknown name.
The relevant part of the spec is
"If a name is given as a <custom-ident>, only lines with that name
are counted. If not enough lines with that name exist,
all implicit grid lines are assumed to have that name
for the purpose of finding this position."
Modified the code to resolve named grid lines in GridResolvedPosition.
We need to keep the old behavior of considering "auto" unknown named
grid lines for the case of positioned grid items.
Updated current tests to the new expected behavior and created a new
test checking different cases explicitly.