[GridNG] Use specified placement and initial out-of-flow support

Previously, grid items were placed according to document order in
PlaceGridItems() in the following change:
https://chromium-review.googlesource.com/c/chromium/src/+/2369673. Now
that the computed grid item positions are available from
https://chromium-review.googlesource.com/c/chromium/src/+/2404110, the
function was refactored to use the final specified/resolved positions
instead of document order.

Furthermore, out of flow items are now being filtered out and stored in
the vector out_of_flow_items_. The look up/calculation for the absolute
and static positioned offset remains a TODO for now.

Bug: 1045599
Change-Id: Ia3ad951983798d6b34f13ef80850736dd374aee8
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2444451
Commit-Queue: Ana Sollano Kim <ansollan@microsoft.com>
Reviewed-by: Christian Biesinger <cbiesinger@chromium.org>
Reviewed-by: Ethan Jimenez <ethavar@microsoft.com>
Reviewed-by: Ian Kilpatrick <ikilpatrick@chromium.org>
Cr-Commit-Position: refs/heads/master@{#819092}

third_party/blink/renderer/core/layout/ng/grid/ng_grid_layout_algorithm.h

@@ -93,7 +93,7 @@ class CORE_EXPORT NGGridLayoutAlgorithm
         : public std::iterator<std::input_iterator_tag, GridItemData> {
      public:
       Iterator(Vector<wtf_size_t>::const_iterator current_index,
-               Vector<GridItemData>* items);
+               Vector<GridItemData>* grid_items);
 
       bool operator!=(const Iterator& other) const;
       GridItemData* operator->();
@@ -102,17 +102,17 @@ class CORE_EXPORT NGGridLayoutAlgorithm
 
      private:
       Vector<wtf_size_t>::const_iterator current_index_;
-      Vector<GridItemData>* items_;
+      Vector<GridItemData>* grid_items_;
     };
 
     ReorderedGridItems(const Vector<wtf_size_t>& reordered_item_indices,
-                       Vector<GridItemData>& items);
+                       Vector<GridItemData>& grid_items);
     Iterator begin();
     Iterator end();
 
    private:
     const Vector<wtf_size_t>& reordered_item_indices_;
-    Vector<GridItemData>& items_;
+    Vector<GridItemData>& grid_items_;
   };
 
   ReorderedGridItems GetReorderedGridItems();
@@ -174,6 +174,12 @@ class CORE_EXPORT NGGridLayoutAlgorithm
 
   // Lays out and computes inline and block offsets for grid items.
   void PlaceGridItems();
+
+  // Lays out |grid_item| based on the offsets and sizes provided.
+  void PlaceGridItem(const GridItemData& grid_item,
+                     LogicalOffset offset,
+                     LogicalSize size);
+
   // Gets the row or column gap of the grid.
   LayoutUnit GridGap(GridTrackSizingDirection track_direction);
 
@@ -183,7 +189,8 @@ class CORE_EXPORT NGGridLayoutAlgorithm
   LogicalSize border_box_size_;
   LogicalSize child_percentage_size_;
 
-  Vector<GridItemData> items_;
+  Vector<GridItemData> grid_items_;
+  Vector<GridItemData> out_of_flow_items_;
   Vector<wtf_size_t> reordered_item_indices_;
 
   NGGridBlockTrackCollection block_column_track_collection_;