LibWeb: Bring CSS line-height closer to other engines

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.

Userland/Libraries/LibWeb/CSS/Length.h

@@ -72,7 +72,7 @@ public:
     };
 
     struct FontMetrics {
-        FontMetrics(CSSPixels font_size, Gfx::FontPixelMetrics const&, CSSPixels line_height);
+        FontMetrics(CSSPixels font_size, Gfx::FontPixelMetrics const&);
 
         CSSPixels font_size;
         CSSPixels x_height;