LibWeb: Use fit-content width in place of indefinite flex item widths

In `flex-direction: column` layouts, a flex item's intrinsic height may
depend on its width, but the width is calculated *after* the intrinsic
height is required.

Unfortunately, the specification doesn't tell us exactly what to do here
(missing inputs to intrinsic sizing is a common problem) so we take the
solution that flexbox applies in 9.2.3.C and apply it to all intrinsic
height calculations within FlexFormattingContext: if the used width of
an item is not yet known when its intrinsic height is requested, we
substitute the fit-content width instead.

Note that while this is technically ad-hoc, it's basically extrapolating
the spec's suggestion in one specific case and using it in all cases.

Userland/Libraries/LibWeb/Layout/FlexFormattingContext.h

@@ -156,7 +156,6 @@ private:
 
     void determine_available_space_for_items(AvailableSpace const&);
 
-    CSSPixels calculate_indefinite_main_size(FlexItem const&);
     void determine_flex_base_size_and_hypothetical_main_size(FlexItem&);
 
     void determine_main_size_of_flex_container();