LibWeb: Assign hypothetical flex item main sizes as temporary main size

This colors a bit outside the lines of the specification, but the spec
doesn't offer a proper explanation for how descendants of a flex item
are supposed to have access to the flex item's main size for purposes
of percentage resolution.

The approach I came up with here was to take the hypothetical main size
of each flex item, and assign it as a temporary main size. This allows
percentage resolution in descendants to work against the pre-flexing
main size of items. This seems to match how other engines behave,
although it feels somewhat dirty. If/when we learn more about this,
we can come up with something nicer.

Userland/Libraries/LibWeb/Layout/LayoutState.cpp

@@ -301,6 +301,16 @@ void LayoutState::UsedValues::set_content_height(float height)
     m_has_definite_height = true;
 }
 
+void LayoutState::UsedValues::set_temporary_content_width(float width)
+{
+    m_content_width = width;
+}
+
+void LayoutState::UsedValues::set_temporary_content_height(float height)
+{
+    m_content_height = height;
+}
+
 float LayoutState::resolved_definite_width(Box const& box) const
 {
     return get(box).content_width();