Move ScrollbarManager from core to platform and use as Scrollbar parent

third_party/WebKit/Source/platform/scroll/Scrollbar.h

Index: third_party/WebKit/Source/platform/scroll/Scrollbar.h
diff --git a/third_party/WebKit/Source/platform/scroll/Scrollbar.h b/third_party/WebKit/Source/platform/scroll/Scrollbar.h
index ecb2b7bc61dcf1aeb7f368d621c934cff1c82b5d..68bc479cfe84e32986e9baa47f460adb15148e8d 100644
--- a/third_party/WebKit/Source/platform/scroll/Scrollbar.h
+++ b/third_party/WebKit/Source/platform/scroll/Scrollbar.h
@@ -26,11 +26,11 @@
 #ifndef Scrollbar_h
 #define Scrollbar_h
 
-#include "platform/FrameViewBase.h"
 #include "platform/Timer.h"
 #include "platform/graphics/paint/DisplayItem.h"
 #include "platform/heap/Handle.h"
 #include "platform/scroll/ScrollTypes.h"
+#include "platform/scroll/ScrollbarManager.h"
 #include "platform/scroll/ScrollbarThemeClient.h"
 #include "platform/wtf/MathExtras.h"
 
@@ -75,8 +75,8 @@ class PLATFORM_EXPORT Scrollbar : public GarbageCollectedFinalized<Scrollbar>,
   IntSize Size() const override { return frame_rect_.Size(); }
   IntPoint Location() const override { return frame_rect_.Location(); }
 
-  virtual void SetParent(FrameViewBase* parent) { parent_ = parent; }
-  FrameViewBase* Parent() const { return parent_; }
+  virtual void SetParent(ScrollbarManager* parent) { parent_ = parent; }
+  ScrollbarManager* Parent() const { return parent_; }
 
   void SetFrameRect(const IntRect&);
   IntRect FrameRect() const override { return frame_rect_; }
@@ -252,7 +252,7 @@ class PLATFORM_EXPORT Scrollbar : public GarbageCollectedFinalized<Scrollbar>,
   bool track_needs_repaint_;
   bool thumb_needs_repaint_;
   LayoutRect visual_rect_;
-  Member<FrameViewBase> parent_;
+  Member<ScrollbarManager> parent_;
   IntRect frame_rect_;
 };