[CSS Grid Layout] Grid items must set a new formatting context.

Source/core/rendering/RenderBox.h

Index: Source/core/rendering/RenderBox.h
diff --git a/Source/core/rendering/RenderBox.h b/Source/core/rendering/RenderBox.h
index 5a899c0b52e8d3b42b418f3e092217a3bf722d12..2e0e89502e2f3eb9f9422f2017403ac00d2733f4 100644
--- a/Source/core/rendering/RenderBox.h
+++ b/Source/core/rendering/RenderBox.h
@@ -530,6 +530,8 @@ public:
     bool isDeprecatedFlexItem() const { return !isInline() && !isFloatingOrOutOfFlowPositioned() && parent() && parent()->isDeprecatedFlexibleBox(); }
     bool isFlexItemIncludingDeprecated() const { return !isInline() && !isFloatingOrOutOfFlowPositioned() && parent() && parent()->isFlexibleBoxIncludingDeprecated(); }
 
+    bool isGridItem() const { return parent() && parent()->isRenderGrid(); }
+
     virtual LayoutUnit lineHeight(bool firstLine, LineDirectionMode, LinePositionMode = PositionOnContainingLine) const override;
     virtual int baselinePosition(FontBaseline, bool firstLine, LineDirectionMode, LinePositionMode = PositionOnContainingLine) const override;