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 4eb861390945b560eef81cecbd34962362a71cf0..7e1d0e96d97c820972880f796c11883969010be5 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)
@@ -350,6 +357,10 @@ private:
     LayoutUnit computedCSSPadding(const Length&) const;
     bool isBoxModelObject() const final { return true; }
 
+    // Returns the bounding rectangle to be used when determining sticky positioning offset. This is the frame rect
+    // for box objects and the lines bounding box for inline objects.
+    virtual LayoutRect frameRectForStickyPositioning() const = 0;
+
     // The PaintLayer associated with this object.
     // |m_layer| can be nullptr depending on the return value of layerTypeRequired().
     OwnPtr<PaintLayer> m_layer;