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 5ba25a8fdee4bff2c47eab9de4d093e6609fadee..c9bc87ef4fc426708f524ffd68dd94fce1716073 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
@@ -341,6 +352,10 @@ public:
     bool shouldRebuildVerticalScrollbarLayer() const { return m_rebuildVerticalScrollbarLayer; }
     void resetRebuildScrollbarLayerFlags();
 
+    StickyConstraintsMap& stickyConstraintsMap() { return ensureRareData().m_stickyConstraintsMap; }
+    void invalidateAllStickyConstraints();
+    void invalidateStickyConstraintsFor(PaintLayer*, bool needsCompositingUpdate = true);
+
     DECLARE_VIRTUAL_TRACE();
 
 private:
@@ -376,6 +391,18 @@ 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(); }
 
@@ -422,6 +449,8 @@ private:
 
     ScrollAnchor m_scrollAnchor;
 
+    OwnPtr<PaintLayerScrollableAreaRareData> m_rareData;
+
 #if ENABLE(ASSERT)
     bool m_hasBeenDisposed;
 #endif