Browsers such as Chrome and Firefox apply an arbitrary scale to the
current font size if `normal` is used for `line-height`. Firefox uses
1.2 while Chrome uses 1.15. Let's go with the latter for now, it's
relatively easy to change if we ever want to go back on that decision.
This also requires updating the expectations for a lot of layout tests.
The upside of this is that it's a bit easier to compare our layout
results to other browsers', especially Chrome.
There's a specific (and thankfully very common!) scenario where we can
actually skip calculating the automatic minimum size for flex items.
In single-line (no wrapping) flex containers, if the sum of all item
flex base sizes is <= the flex container's main size, we know that
none of the items will be shrunk by the layout algorithm.
And so for any flex item with definite main size AND automatic minimum
main size, we can treat the automatic minimum size as 0.
