Move ScrollbarManager from core to platform and use as Scrollbar parent

third_party/WebKit/Source/platform/BUILD.gn

 # Copyright 2014 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("//build/config/features.gni")
 import("//build/config/ui.gni")
 import("//testing/libfuzzer/fuzzer_test.gni")
 import("//testing/test.gni")
 import("//third_party/WebKit/public/public_features.gni")
 import("//third_party/WebKit/Source/build/scripts/scripts.gni")
@@ skipping 1287 matching lines @@
     "scroll/ScrollAnimatorBase.cpp",
     "scroll/ScrollAnimatorBase.h",
     "scroll/ScrollAnimatorCompositorCoordinator.cpp",
     "scroll/ScrollAnimatorCompositorCoordinator.h",
     "scroll/ScrollStateData.h",
     "scroll/ScrollTypes.h",
     "scroll/ScrollableArea.cpp",
     "scroll/ScrollableArea.h",
     "scroll/Scrollbar.cpp",
     "scroll/Scrollbar.h",
+    "scroll/ScrollbarManager.cpp",
+    "scroll/ScrollbarManager.h",
     "scroll/ScrollbarTheme.cpp",
     "scroll/ScrollbarTheme.h",
     "scroll/ScrollbarThemeAndroid.cpp",
     "scroll/ScrollbarThemeAura.cpp",
     "scroll/ScrollbarThemeAura.h",
     "scroll/ScrollbarThemeClient.h",
     "scroll/ScrollbarThemeMac.h",
     "scroll/ScrollbarThemeMac.mm",
     "scroll/ScrollbarThemeMock.cpp",
     "scroll/ScrollbarThemeMock.h",
@@ skipping 850 matching lines @@
     "//third_party/WebKit/Source:config",
     "//third_party/WebKit/Source:inside_blink",
   ]
 
   deps = [
     ":test_support",
     "//testing/gmock",
     "//testing/gtest",
   ]
 }