[css-inline-3] leading-trim and descendant inlines

Currently leading-trim only looks at the root inline box's metric. What if there is content on the first/last line that is taller than the root inline? Should that content also be considered?

We might want to add safe/unsafe keywords to leading-trim, to look at the descendants and pick the highest one, or not and just match to the root inline box. (Probably the default should be matching the root inline box, though.)