LibWeb: Use FlexItem& item consistenly in FlexFormattingContext

There was a pointless mix of `FlexItem*` vs `FlexItem&` as well as
`flex_item` vs `item`. Let's pick one and be consistent.

Userland/Libraries/LibWeb/Layout/FlexFormattingContext.h

@@ -87,7 +87,7 @@ private:
     };
 
     struct FlexLine {
-        Vector<FlexItem*> items;
+        Vector<FlexItem&> items;
         CSSPixels cross_size { 0 };
         CSSPixels remaining_free_space { 0 };
         float chosen_flex_fraction { 0 };