To better distinguish the calculation of line height (still present with args)
and simply getting the line height without args, it's now called GetLineHeight()
This also introduces `mLineHeight` to cache specifically calculated line heights
that aren't "auto" (which is a magic value), and it opens up the possibility to
override it in Part 7.
This is just a clean port of 1322191 and follow-up 1325970. It really seems to add create a new way to access existing code relating to block formatting and floating elements rather than implementing new functionality, and it is mercifully straightforwards.
