[Grid] Serialize grid-areas for implict tracks in grid-template values

This CL fixes an issue with serializing the `grid-template` shorthand
when it is defined via longhands that create implicit tracks. The
current behavior iterates over explicit track sizes, which leaves out
grid areas that cover implicit tracks. The fix is to iterate over the
remaining rows defined in the grid-area map to ensure all defined
grid-areas are serialized. This will ensure that `grid-template`
properly round-trips without dropping grid area definitions.

WPT coverage was very light for `grid-template` shorthands with
implicit tracks, so I augmented a few existing tests to cover these
scenarios. No additional issues were discovered. There weren't any
tests explicitly testing the `grid-template` serialization via
longhands, so I added a new test that fails without this fix
(grid-template-shorthand-composition.html).

Change-Id: If88460391fbfa2127cf07f32e7b81acb6ece6536
Bug: 380369291
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6068810
Reviewed-by: Alison Maher <almaher@microsoft.com>
Reviewed-by: Ethan Jimenez <ethavar@microsoft.com>
Commit-Queue: Kurt Catti-Schmidt <kschmi@microsoft.com>
Cr-Commit-Position: refs/heads/main@{#1392387}