Adapt and reland old position sticky implementation

third_party/WebKit/Source/core/paint/PaintLayerScrollableArea.h

Index: third_party/WebKit/Source/core/paint/PaintLayerScrollableArea.h
diff --git a/third_party/WebKit/Source/core/paint/PaintLayerScrollableArea.h b/third_party/WebKit/Source/core/paint/PaintLayerScrollableArea.h
index ebd01e1c347157c9bcc6d275efdeee30a85b7051..a89baf3a0edce423476ba93e26ea6d9c2cb35c6d 100644
--- a/third_party/WebKit/Source/core/paint/PaintLayerScrollableArea.h
+++ b/third_party/WebKit/Source/core/paint/PaintLayerScrollableArea.h
@@ -63,6 +63,17 @@ class LayoutBox;
 class PaintLayer;
 class LayoutScrollbarPart;
 
+typedef WTF::HashMap<PaintLayer*, StickyPositionScrollingConstraints> StickyConstraintsMap;
+
+struct PaintLayerScrollableAreaRareData {
+    WTF_MAKE_NONCOPYABLE(PaintLayerScrollableAreaRareData);
+    USING_FAST_MALLOC(PaintLayerScrollableAreaRareData);
+public:
+    PaintLayerScrollableAreaRareData() {}
+
+    StickyConstraintsMap m_stickyConstraintsMap;
+};
+
 // PaintLayerScrollableArea represents the scrollable area of a LayoutBox.
 //
 // To be scrollable, an element requires ‘overflow’ != visible. Note that this
@@ -337,6 +348,9 @@ public:
     ScrollAnchor& scrollAnchor() { return m_scrollAnchor; }
     bool isPaintLayerScrollableArea() const override { return true; }
 
+    StickyConstraintsMap& stickyConstraintsMap() { return ensureRareData().m_stickyConstraintsMap; }
+    void invalidateStickyConstraints();
+
     DECLARE_VIRTUAL_TRACE();
 
 private:
@@ -373,6 +387,18 @@ private:
 
     void updateCompositingLayersAfterScroll();
 
+    PaintLayerScrollableAreaRareData* rareData()
+    {
+        return m_rareData.get();
+    }
+
+    PaintLayerScrollableAreaRareData& ensureRareData()
+    {
+        if (!m_rareData)
+            m_rareData = adoptPtr(new PaintLayerScrollableAreaRareData());
+        return *m_rareData.get();
+    }
+
     // PaintInvalidationCapableScrollableArea
     LayoutBox& boxForScrollControlPaintInvalidation() const { return box(); }
 
@@ -413,6 +439,8 @@ private:
 
     ScrollAnchor m_scrollAnchor;
 
+    OwnPtr<PaintLayerScrollableAreaRareData> m_rareData;
+
 #if ENABLE(ASSERT)
     bool m_hasBeenDisposed;
 #endif