[css-rhythm][css-line-grid] Work out the interaction between line-height-step and line-grid

[frivoal]

If we're not careful, the line-height-step property and the line grid will interfere instead of complementing each other.

The grid created byline-grid:create needs to take line-height-step into account. As far as I can tell, the current wording already does that implicitly, but we should probably make it explicit to avoid any misunderstanding.

[frivoal]

We resolved in favor of that: https://logs.csswg.org/irc.w3.org/css/2017-01-12/#e760061