Move ScrollbarManager from core to platform and use as Scrollbar parent

third_party/WebKit/Source/core/paint/BUILD.gn

 # Copyright 2016 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 import("//third_party/WebKit/Source/core/core.gni")
 
 blink_core_sources("paint") {
   split_count = 2
   sources = [
     "BackgroundImageGeometry.cpp",
@@ skipping 150 matching lines @@
     "SVGRootPainter.cpp",
     "SVGRootPainter.h",
     "SVGShapePainter.cpp",
     "SVGShapePainter.h",
     "SVGTextPainter.cpp",
     "SVGTextPainter.h",
     "ScrollRecorder.cpp",
     "ScrollRecorder.h",
     "ScrollableAreaPainter.cpp",
     "ScrollableAreaPainter.h",
-    "ScrollbarManager.cpp",
-    "ScrollbarManager.h",
     "ScrollbarPainter.cpp",
     "ScrollbarPainter.h",
     "TableCellPainter.cpp",
     "TableCellPainter.h",
     "TablePaintInvalidator.cpp",
     "TablePaintInvalidator.h",
     "TablePainter.cpp",
     "TablePainter.h",
     "TableRowPainter.cpp",
     "TableRowPainter.h",
@@ skipping 17 matching lines @@
     "ViewPaintInvalidator.h",
     "ViewPainter.cpp",
     "ViewPainter.h",
   ]
 
   configs += [
     # TODO(jschuh): crbug.com/167187 fix size_t to int truncations.
     "//build/config/compiler:no_size_t_to_int_warning",
   ]
 }