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

Source/core/rendering/RenderBox.h

 /*
  * Copyright (C) 1999 Lars Knoll (knoll@kde.org)
  *           (C) 1999 Antti Koivisto (koivisto@kde.org)
  * Copyright (C) 2003, 2006, 2007 Apple Inc. All rights reserved.
  *
  * This library is free software; you can redistribute it and/or
  * modify it under the terms of the GNU Library General Public
  * License as published by the Free Software Foundation; either
  * version 2 of the License, or (at your option) any later version.
  *
@@ skipping 512 matching lines @@
     bool shrinkToAvoidFloats() const;
     virtual bool avoidsFloats() const;
 
     virtual void markForPaginationRelayoutIfNeeded(SubtreeLayoutScope&);
 
     bool isWritingModeRoot() const { return !parent() || parent()->style()->writingMode() != style()->writingMode(); }
 
     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;
 
     virtual LayoutUnit offsetLeft() const override;
     virtual LayoutUnit offsetTop() const override;
 
     LayoutPoint flipForWritingModeForChild(const RenderBox* child, const LayoutPoint&) const;
     LayoutUnit flipForWritingMode(LayoutUnit position) const WARN_UNUSED_RETURN
     {
         // The offset is in the block direction (y for horizontal writing modes, x for vertical writing modes).
@@ skipping 261 matching lines @@
         if (UNLIKELY(inlineBoxWrapper() != 0))
             deleteLineBoxWrapper();
     }
 
     ensureRareData().m_inlineBoxWrapper = boxWrapper;
 }
 
 } // namespace blink
 
 #endif // RenderBox_h