[css-grid-3][masonry] Shift masonry keyword to grid-auto-rows/columns?

[fantasai]

From one of our front-end engineers:

While reading the article, I started to wonder why this is part of grid-template-* instead of grid-auto-*. Wouldn’t masonry fit better with the concept of rows being implicitly created rather than manually specified?

[tabatkins]

This seems somewhat worse than the current design, since it would mean that people can still define a full NxM grid in grid-template, and then declare that one of the axises is ignored in favor of using masonry behavior. Masonry doesn't create any rows, after all.