Adapt and reland old position sticky implementation

third_party/WebKit/Source/core/layout/LayoutBoxModelObject.h

Index: third_party/WebKit/Source/core/layout/LayoutBoxModelObject.h
diff --git a/third_party/WebKit/Source/core/layout/LayoutBoxModelObject.h b/third_party/WebKit/Source/core/layout/LayoutBoxModelObject.h
index 8e55591c00a6c29583c96b888399362ee16b99c9..4003da3ac7c06b9ddf77d0776fac16e52810ac49 100644
--- a/third_party/WebKit/Source/core/layout/LayoutBoxModelObject.h
+++ b/third_party/WebKit/Source/core/layout/LayoutBoxModelObject.h
@@ -62,6 +62,7 @@ enum ContentChangeType {
 };
 
 class InlineFlowBox;
+class StickyPositionViewportConstraints;
 
 // This class is the base class for all CSS objects.
 //
@@ -140,6 +141,12 @@ public:
     LayoutSize relativePositionOffset() const;
     LayoutSize relativePositionLogicalOffset() const { return style()->isHorizontalWritingMode() ? relativePositionOffset() : relativePositionOffset().transposedSize(); }
 
+    // Populates StickyPositionConstraints, setting the sticky box rect, containing block rect and updating
+    // the constraint offsets according to the available space.
+    void computeStickyPositionConstraints(StickyPositionViewportConstraints&, const LayoutRect& constrainingRect) const;
+    LayoutRect computeStickyConstrainingRect() const;
+    LayoutSize stickyPositionOffset() const;
+
     LayoutSize offsetForInFlowPosition() const;
 
     // IE extensions. Used to calculate offsetWidth/Height.  Overridden by inlines (LayoutFlow)