[css-grid] Fix simplified layout of positioned grid items

third_party/WebKit/Source/core/layout/LayoutGrid.cpp

Index: third_party/WebKit/Source/core/layout/LayoutGrid.cpp
diff --git a/third_party/WebKit/Source/core/layout/LayoutGrid.cpp b/third_party/WebKit/Source/core/layout/LayoutGrid.cpp
index 05a01b709fbfee65abbd54397c163e4667c2d148..3a47a2c2fd267c6eae8eeaea5360dcf4222644c0 100644
--- a/third_party/WebKit/Source/core/layout/LayoutGrid.cpp
+++ b/third_party/WebKit/Source/core/layout/LayoutGrid.cpp
@@ -472,6 +472,15 @@ void LayoutGrid::repeatTracksSizingIfNeeded(GridSizingData& sizingData,
   }
 }
 
+bool LayoutGrid::simplifiedLayout() {
+  // This is a way to protect simplifiedLayout(), as we cannot perform it
+  // on a dirty grid that has positioned items to be layout.
+  if (m_gridIsDirty && posChildNeedsLayout())
+    return false;
+
+  return LayoutBlock::simplifiedLayout();
+}
+
 void LayoutGrid::layoutBlock(bool relayoutChildren) {
   ASSERT(needsLayout());