This fixes a plethora of rounding problems on many websites.
In the future, we may want to replace this with fixed-point arithmetic
(bug #18566) for performance (and consistency with other engines),
but in the meantime this makes the web look a bit better. :^)
There's a lot more things that could be converted to doubles, which
would reduce the amount of casting necessary in this patch.
We can do that incrementally, however.
This change addresses the incorrect assumption that the available width
inside a grid item is equal to the width of the track it belongs to.
For instance, if a grid item has a width of 200px, the available width
inside that item is also 200px regardless of its column(s) base size.
To solve this issue, it was necessary to move the final resolution of
grid items to occur immediately after the final column track sizes are
determined. By doing so, it becomes possible to obtain correct
available width inside grid items while resolving the row track sizes.
1. Stop using -1 to indicate infinity value of growth limit. Just use
INFINITY for that.
2. More complete implementation of "Expand Flexible Tracks" step.
3. Return AvailableSize from get_free_space: spec says that this
function can return indefinite size and it is ok.
The file gap.html, which previously had multiple grid tests, has now
been divided into smaller files, each containing only one grid test.
It is going to make it easier to identify what inputs have been
affected by changes in layout code.
