LibWeb: Use correct width for flex item intrinsic height calculation

Before this change, we always used the flex container's full available
space as the width for intrinsic (height) sizing of flex items.

This meant that flex lines with more than one flex item had their
intrinsic height determined as if they were alone on the line.

For flex row layouts, if we've already determined the flex item's main
size, we now use that as the width to get the intrinsic height.

This leads to more correct layouts, and also avoids some redundant work
since we no longer do unnecessary sizing work with the wrong width (and
can hit cache instead).

Libraries/LibWeb/Layout/FlexFormattingContext.cpp

@@ -22,6 +22,14 @@ CSSPixels FlexFormattingContext::get_pixel_width(FlexItem const& item, CSS::Size
 
 CSSPixels FlexFormattingContext::get_pixel_height(FlexItem const& item, CSS::Size const& size) const
 {
+    if (is_row_layout()) {
+        // NOTE: In a row layout, after we've determined the main size, we use that as the available width
+        //       for any intrinsic sizing layout needed to resolve the height.
+        auto available_width = item.main_size.has_value() ? AvailableSize::make_definite(item.main_size.value()) : AvailableSize::make_indefinite();
+        auto available_height = AvailableSize::make_indefinite();
+        auto available_space = AvailableSpace { available_width, available_height };
+        return calculate_inner_height(item.box, available_space, size);
+    }
     return calculate_inner_height(item.box, m_available_space.value(), size);
 }