MacViews: Fix scrolling when clicking on the views::ScrollView scroll track

ui/views/controls/scrollbar/native_scroll_bar.h

 // Copyright (c) 2012 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.
 
 #ifndef UI_VIEWS_CONTROLS_SCROLLBAR_NATIVE_SCROLL_BAR_H_
 #define UI_VIEWS_CONTROLS_SCROLLBAR_NATIVE_SCROLL_BAR_H_
 
 #include <string>
 
 #include "base/compiler_specific.h"
 #include "base/gtest_prod_util.h"
 #include "base/macros.h"
 #include "ui/views/controls/scrollbar/scroll_bar.h"
 #include "ui/views/view.h"
 
 namespace ui {
 class NativeTheme;
 }
 
 namespace views {
+namespace test {
+class ScrollViewTestApi;
+}
 
 class NativeScrollBarWrapper;
 
 // The NativeScrollBar class is a scrollbar that uses platform's
 // native control.
 class VIEWS_EXPORT NativeScrollBar : public ScrollBar {
  public:
   // The scroll-bar's class name.
   static const char kViewClassName[];
 
   // Create new scrollbar, either horizontal or vertical.
   explicit NativeScrollBar(bool is_horiz);
   ~NativeScrollBar() override;
 
   // Return the system sizes.
   static int GetHorizontalScrollBarHeight(const ui::NativeTheme* theme);
   static int GetVerticalScrollBarWidth(const ui::NativeTheme* theme);
 
  private:
   friend class NativeScrollBarTest;
+  friend class test::ScrollViewTestApi;
   FRIEND_TEST_ALL_PREFIXES(NativeScrollBarTest, Scrolling);
 
   // Overridden from View.
   gfx::Size GetPreferredSize() const override;
   void Layout() override;
   void ViewHierarchyChanged(
       const ViewHierarchyChangedDetails& details) override;
   const char* GetClassName() const override;
 
   // Overrideen from View for keyboard UI purpose.
@@ skipping 10 matching lines @@
 
   // init border
   NativeScrollBarWrapper* native_wrapper_;
 
   DISALLOW_COPY_AND_ASSIGN(NativeScrollBar);
 };
 
 }  // namespace views
 
 #endif  // UI_VIEWS_CONTROLS_SCROLLBAR_NATIVE_SCROLL_BAR_H_