Bug
1558783 - Fixes the subgrid and parent grid highlighter states when both are highlighted. r=mtigley
This fixes 2 issues that we had when the subgrid and parent grid highlighter are shown:
1) When the subgrid and its parent grid container are both highlighted and the parent grid container
is unhighlighted, we need to still show a translucent parent grid container highlight.
2) Avoid showing a translucent parent grid container highlight for a subgrid, when the
parent grid container is already highlighted.
Differential Revision: https://phabricator.services.mozilla.com/D35725