This patch makes a few changes to the way we calculate line-height:
- `line-height: normal` is now resolved using metrics from the used
font (specifically, round(A + D + lineGap)).
- `line-height: calc(...)` is now resolved at style compute time.
- `line-height` values are now absolutized at style compute time.
As a consequence of the above, we no longer need to walk the DOM
ancestor chain looking for line-heights during style computation.
Instead, values are inherited, resolved and absolutized locally.
This is not only much faster, but also makes our line-height metrics
match those of other engines like Gecko and Blink.
Fixes following mistakes:
- "scrolling box" for a document is not `scrollable_overflow_rect()`
but size of viewport (initial containing block, like spec says).
- comparing edges of "scrolling box" with edges of target element
does not make any sense because "scrolling box" edges are relative
to page while result of `get_bounding_client_rect()` is relative
to viewport.
