Revert of Avoid scrollbar construction/destruction thrashing during flex layout.

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

Index: third_party/WebKit/Source/core/paint/DeprecatedPaintLayerScrollableArea.h
diff --git a/third_party/WebKit/Source/core/paint/DeprecatedPaintLayerScrollableArea.h b/third_party/WebKit/Source/core/paint/DeprecatedPaintLayerScrollableArea.h
index f9823dae6a16c5c29011784a661c0903020bc1a1..cb28579ff697d3220dd7284a72f1124fd6b7083d 100644
--- a/third_party/WebKit/Source/core/paint/DeprecatedPaintLayerScrollableArea.h
+++ b/third_party/WebKit/Source/core/paint/DeprecatedPaintLayerScrollableArea.h
@@ -67,53 +67,6 @@
     WILL_BE_USING_GARBAGE_COLLECTED_MIXIN(DeprecatedPaintLayerScrollableArea);
     friend class Internals;
 
-private:
-    class ScrollbarManager {
-        // Helper class to manage the life cycle of Scrollbar objects.  Some layout containers
-        // (e.g., flexbox, table) run multi-pass layout on their children, applying different
-        // constraints.  If a child has overflow:auto, it may gain and lose scrollbars multiple
-        // times during multi-pass layout, causing pointless allocation/deallocation thrashing,
-        // and potentially leading to other problems (crbug.com/528940).
-
-        // ScrollbarManager allows a ScrollableArea to delay the destruction of a scrollbar that
-        // is no longer needed, until the end of multi-pass layout.  If the scrollbar is then
-        // re-added before multi-pass layout finishes, the previously "deleted" scrollbar will
-        // be restored, rather than constructing a new one.
-    public:
-        ScrollbarManager(DeprecatedPaintLayerScrollableArea&);
-
-        void dispose();
-
-        // When canDetachScrollbars is true, calls to setHas*Scrollbar(false) will NOT destroy
-        // an existing scrollbar, but instead detach it without destroying it.  If, subsequently,
-        // setHas*Scrollbar(true) is called, the existing scrollbar will be reattached.  When
-        // setCanDetachScrollbars(false) is called, any detached scrollbars will be destructed.
-        bool canDetachScrollbars() const { return m_canDetachScrollbars; }
-        void setCanDetachScrollbars(bool);
-
-        Scrollbar* horizontalScrollbar() const { return m_hBarIsAttached ? m_hBar.get(): nullptr; }
-        Scrollbar* verticalScrollbar() const { return m_vBarIsAttached ? m_vBar.get() : nullptr; }
-        bool hasHorizontalScrollbar() const { return horizontalScrollbar(); }
-        bool hasVerticalScrollbar() const { return verticalScrollbar(); }
-
-        void setHasHorizontalScrollbar(bool hasScrollbar);
-        void setHasVerticalScrollbar(bool hasScrollbar);
-
-        DECLARE_TRACE();
-
-    private:
-        PassRefPtrWillBeRawPtr<Scrollbar> createScrollbar(ScrollbarOrientation);
-        void destroyScrollbar(ScrollbarOrientation, bool invalidate = false);
-
-    private:
-        DeprecatedPaintLayerScrollableArea& m_scrollableArea;
-        RefPtrWillBeMember<Scrollbar> m_hBar;
-        RefPtrWillBeMember<Scrollbar> m_vBar;
-        unsigned m_canDetachScrollbars: 1;
-        unsigned m_hBarIsAttached: 1;
-        unsigned m_vBarIsAttached: 1;
-    };
-
 public:
     // FIXME: We should pass in the LayoutBox but this opens a window
     // for crashers during DeprecatedPaintLayer setup (see crbug.com/368062).
@@ -128,8 +81,8 @@
     bool hasHorizontalScrollbar() const { return horizontalScrollbar(); }
     bool hasVerticalScrollbar() const { return verticalScrollbar(); }
 
-    Scrollbar* horizontalScrollbar() const override { return m_scrollbarManager.horizontalScrollbar(); }
-    Scrollbar* verticalScrollbar() const override { return m_scrollbarManager.verticalScrollbar(); }
+    Scrollbar* horizontalScrollbar() const override { return m_hBar.get(); }
+    Scrollbar* verticalScrollbar() const override { return m_vBar.get(); }
 
     HostWindow* hostWindow() const override;
 
@@ -209,7 +162,7 @@
 
     bool updateAfterCompositingChange() override;
 
-    bool hasScrollbar() const { return hasHorizontalScrollbar() || hasVerticalScrollbar(); }
+    bool hasScrollbar() const { return m_hBar || m_vBar; }
 
     LayoutScrollbarPart* scrollCorner() const { return m_scrollCorner; }
 
@@ -301,6 +254,9 @@
     LayoutUnit horizontalScrollbarStart(int minX) const;
     IntSize scrollbarOffset(const Scrollbar*) const;
 
+    PassRefPtrWillBeRawPtr<Scrollbar> createScrollbar(ScrollbarOrientation);
+    void destroyScrollbar(ScrollbarOrientation);
+
     void setHasHorizontalScrollbar(bool hasScrollbar);
     void setHasVerticalScrollbar(bool hasScrollbar);
 
@@ -333,13 +289,14 @@
     // The width/height of our scrolled area.
     LayoutRect m_overflowRect;
 
-    // ScrollbarManager holds the Scrollbar instances.
-    ScrollbarManager m_scrollbarManager;
-
     // This is the (scroll) offset from scrollOrigin().
     DoubleSize m_scrollOffset;
 
     IntPoint m_cachedOverlayScrollbarOffset;
+
+    // For areas with overflow, we have a pair of scrollbars.
+    RefPtrWillBeMember<Scrollbar> m_hBar;
+    RefPtrWillBeMember<Scrollbar> m_vBar;
 
     // LayoutObject to hold our custom scroll corner.
     LayoutScrollbarPart* m_scrollCorner;