Reland main thread 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 bd836e914f05ad4b710539c501345579d49f948c..39f95eb3411d27686daa28630d0ae5515bccf7eb 100644
--- a/third_party/WebKit/Source/core/layout/LayoutBoxModelObject.h
+++ b/third_party/WebKit/Source/core/layout/LayoutBoxModelObject.h
@@ -26,6 +26,7 @@
 
 #include "core/CoreExport.h"
 #include "core/layout/LayoutObject.h"
+#include "core/page/scrolling/StickyPositionScrollingConstraints.h"
 #include "core/style/ShadowData.h"
 #include "platform/geometry/LayoutRect.h"
 
@@ -64,6 +65,15 @@ enum ContentChangeType {
 
 class InlineFlowBox;
 
+struct LayoutBoxModelObjectRareData {
+    WTF_MAKE_NONCOPYABLE(LayoutBoxModelObjectRareData);
+    USING_FAST_MALLOC(LayoutBoxModelObjectRareData);
+public:
+    LayoutBoxModelObjectRareData() {}
+
+    StickyPositionScrollingConstraints m_stickyPositionScrollingConstraints;
+};
+
 // This class is the base class for all CSS objects.
 //
 // All CSS objects follow the box model object. See THE BOX MODEL section in
@@ -141,6 +151,12 @@ 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.
+    FloatRect computeStickyConstrainingRect() const;
+    void updateStickyPositionConstraints() const;
+    LayoutSize stickyPositionOffset() const;
+
     LayoutSize offsetForInFlowPosition() const;
 
     // IE extensions. Used to calculate offsetWidth/Height.  Overridden by inlines (LayoutFlow)
@@ -330,6 +346,8 @@ protected:
     void styleWillChange(StyleDifference, const ComputedStyle& newStyle) override;
     void styleDidChange(StyleDifference, const ComputedStyle* oldStyle) override;
 
+    void invalidateStickyConstraints();
+
 public:
     // These functions are only used internally to manipulate the layout tree structure via remove/insert/appendChildNode.
     // Since they are typically called only to move objects around within anonymous blocks (which only have layers in
@@ -361,9 +379,18 @@ private:
     LayoutUnit computedCSSPadding(const Length&) const;
     bool isBoxModelObject() const final { return true; }
 
+    LayoutBoxModelObjectRareData& ensureRareData()
+    {
+        if (!m_rareData)
+            m_rareData = adoptPtr(new LayoutBoxModelObjectRareData());
+        return *m_rareData.get();
+    }
+
     // The PaintLayer associated with this object.
     // |m_layer| can be nullptr depending on the return value of layerTypeRequired().
     OwnPtr<PaintLayer> m_layer;
+
+    OwnPtr<LayoutBoxModelObjectRareData> m_rareData;
 };
 
 DEFINE_LAYOUT_OBJECT_TYPE_CASTS(LayoutBoxModelObject, isBoxModelObject());