[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..d22825532de58c6611bad2c0e47d683c8e8ef83f 100644
--- a/third_party/WebKit/Source/core/layout/LayoutGrid.cpp
+++ b/third_party/WebKit/Source/core/layout/LayoutGrid.cpp
@@ -475,7 +475,10 @@ void LayoutGrid::repeatTracksSizingIfNeeded(GridSizingData& sizingData,
 void LayoutGrid::layoutBlock(bool relayoutChildren) {
   ASSERT(needsLayout());
 
-  if (!relayoutChildren && simplifiedLayout())
+  // We cannot perform a simplifiedLayout() on a dirty grid that
+  // has positioned items to be laid out.
+  if (!relayoutChildren && (!m_gridIsDirty || !posChildNeedsLayout()) &&
+      simplifiedLayout())
     return;
 
   SubtreeLayoutScope layoutScope(*this);