[CSS Grid Layout] Ignore positioned block item added after inline

Source/core/rendering/RenderGrid.cpp

Index: Source/core/rendering/RenderGrid.cpp
diff --git a/Source/core/rendering/RenderGrid.cpp b/Source/core/rendering/RenderGrid.cpp
index e7daaecd24e539d34fa4095dad4fb805159dccc8..8e4c9146bd1dace75fb64d23cb75eb5efc1fd2b2 100644
--- a/Source/core/rendering/RenderGrid.cpp
+++ b/Source/core/rendering/RenderGrid.cpp
@@ -219,6 +219,11 @@ void RenderGrid::addChild(RenderObject* newChild, RenderObject* beforeChild)
         return;
     }
 
+    // If the new child has been inserted inside an existent anonymous block, we can simply ignore it as the anonymous
+    // block is an already known grid item.
+    if (newChild->parent() != this)
+        return;
+
     // FIXME: Implement properly "stack" value in auto-placement algorithm.
     if (!style()->isGridAutoFlowAlgorithmStack()) {
         // The grid needs to be recomputed as it might contain auto-placed items that will change their position.