Mac: Update scrollbar size and input position during impl-overscroll

Source/platform/scroll/Scrollbar.h

Index: Source/platform/scroll/Scrollbar.h
diff --git a/Source/platform/scroll/Scrollbar.h b/Source/platform/scroll/Scrollbar.h
index 625412d2f025f2684e511c79e5569e2a383464f8..8fcdb5f856b71200b088fdd81e1ff27fb0cb12d5 100644
--- a/Source/platform/scroll/Scrollbar.h
+++ b/Source/platform/scroll/Scrollbar.h
@@ -144,6 +144,9 @@ public:
     virtual bool isAlphaLocked() const override { return m_isAlphaLocked; }
     virtual void setIsAlphaLocked(bool flag) override { m_isAlphaLocked = flag; }
 
+    virtual float elasticOverscroll() const override { return m_elasticOverscroll; }
+    virtual void setElasticOverscroll(float elasticOverscroll) override { m_elasticOverscroll = elasticOverscroll; }
+
     bool overlapsResizer() const { return m_overlapsResizer; }
     void setOverlapsResizer(bool overlapsResizer) { m_overlapsResizer = overlapsResizer; }
 
@@ -196,6 +199,8 @@ protected:
 
     bool m_isAlphaLocked;
 
+    float m_elasticOverscroll;
+
 private:
     virtual bool isScrollbar() const override { return true; }