[CSS Grid Layout] Fix positioned grid children position and size

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 0dc27147f21e9d0e97c9cd9a70a68b0612504eb4..f442c665218d8cbc25be33ecebf699b00365ad15 100644
--- a/Source/core/rendering/RenderBox.h
+++ b/Source/core/rendering/RenderBox.h
@@ -183,7 +183,9 @@ public:
 
     // Use this with caution! No type checking is done!
     RenderBox* previousSiblingBox() const;
+    RenderBox* previousInFlowSiblingBox() const;
     RenderBox* nextSiblingBox() const;
+    RenderBox* nextInFlowSiblingBox() const;
     RenderBox* parentBox() const;
 
     bool canResize() const;
@@ -349,6 +351,14 @@ public:
     void clearContainingBlockOverrideSize();
     void clearOverrideContainingBlockContentLogicalHeight();
 
+    LayoutUnit overrideInlineOffset() const;
+    LayoutUnit overrideBlockOffset() const;
+    bool hasOverrideInlineOffset() const;
+    bool hasOverrideBlockOffset() const;
+    void setOverrideInlineOffset(LayoutUnit inlineOffest);
+    void setOverrideBlockOffset(LayoutUnit blockOffest);
+    void clearOverrideInlineAndBlockOffests();
+
     virtual LayoutSize offsetFromContainer(const RenderObject*, const LayoutPoint&, bool* offsetDependsOnPoint = 0) const override;
 
     LayoutUnit adjustBorderBoxLogicalWidthForBoxSizing(LayoutUnit width) const;
@@ -774,11 +784,27 @@ inline RenderBox* RenderBox::previousSiblingBox() const
     return toRenderBox(previousSibling());
 }
 
+inline RenderBox* RenderBox::previousInFlowSiblingBox() const
+{
+    RenderBox* previous = previousSiblingBox();
+    while (previous && previous->isOutOfFlowPositioned())
+        previous = previous->previousSiblingBox();
+    return previous;
+}
+
 inline RenderBox* RenderBox::nextSiblingBox() const
 {
     return toRenderBox(nextSibling());
 }
 
+inline RenderBox* RenderBox::nextInFlowSiblingBox() const
+{
+    RenderBox* next = nextSiblingBox();
+    while (next && next->isOutOfFlowPositioned())
+        next = next->nextSiblingBox();
+    return next;
+}
+
 inline RenderBox* RenderBox::parentBox() const
 {
     return toRenderBox(parent());