Bug 1341802 - stylo: Enable the forgotten disabled grid test r=me

MozReview-Commit-ID: 6ZrPuFigxGL

layout/style/test/mochitest.ini

@@ -219,7 +219,6 @@ support-files =
 [test_grid_item_shorthands.html]
 [test_grid_shorthand_serialization.html]
 [test_grid_computed_values.html]
-skip-if = stylo # bug 1339656
 [test_group_insertRule.html]
 [test_hover_quirk.html]
 [test_html_attribute_computed_values.html]