Bug 1009776 - part 3, [css-grid] Store the grid item's grid area on a frame property. r=dholbert

layout/generic/nsGridContainerFrame.cpp

@@ -438,6 +438,12 @@ nsGridContainerFrame::PlaceGridItems(const nsStylePosition* aStyle)
   for (nsFrameList::Enumerator e(PrincipalChildList()); !e.AtEnd(); e.Next()) {
     nsIFrame* child = e.get();
     const GridArea& area = PlaceDefinite(child, aStyle);
+    GridArea* prop = GetGridAreaForChild(child);
+    if (prop) {
+      *prop = area;
+    } else {
+      child->Properties().Set(GridAreaProperty(), new GridArea(area));
+    }
     if (area.IsDefinite()) {
       InflateGridFor(area);
     }

layout/generic/nsGridContainerFrame.h

@@ -203,6 +203,15 @@ protected:
     return areas && areas->Contains(aName);
   }
 
+  NS_DECLARE_FRAME_PROPERTY(GridAreaProperty, DeleteValue<GridArea>)
+
+  /**
+   * A convenience method to get the stored GridArea* for a frame.
+   */
+  static GridArea* GetGridAreaForChild(nsIFrame* aChild) {
+    return static_cast<GridArea*>(aChild->Properties().Get(GridAreaProperty()));
+  }
+
 #ifdef DEBUG
   void SanityCheckAnonymousGridItems() const;
 #endif // DEBUG