MacViews: Reveal scrollbars when resting on the trackpad.

ui/views/controls/scrollbar/cocoa_scroll_bar.h

Index: ui/views/controls/scrollbar/cocoa_scroll_bar.h
diff --git a/ui/views/controls/scrollbar/cocoa_scroll_bar.h b/ui/views/controls/scrollbar/cocoa_scroll_bar.h
index 2cca52a6e380aa0400baf2b0c70ec5152762b9eb..8b91f6c3cde2f274d60d020ce492056adc20f4fd 100644
--- a/ui/views/controls/scrollbar/cocoa_scroll_bar.h
+++ b/ui/views/controls/scrollbar/cocoa_scroll_bar.h
@@ -26,8 +26,11 @@ class VIEWS_EXPORT CocoaScrollBar : public BaseScrollBar,
   explicit CocoaScrollBar(bool horizontal);
   ~CocoaScrollBar() override;
 
-  // BaseScrollBar:
-  void ScrollToPosition(int position) override;
+  // ScrollBar:
+  void Update(int viewport_size,
+              int content_size,
+              int contents_scroll_offset) override;
+  void ObserveScrollEvent(const ui::ScrollEvent& event) override;
 
   // ViewsScrollbarBridgeDelegate:
   void OnScrollerStyleChanged() override;
@@ -65,6 +68,8 @@ class VIEWS_EXPORT CocoaScrollBar : public BaseScrollBar,
   void OnPaint(gfx::Canvas* canvas) override;
 
  private:
+  friend class BaseScrollBar;  // For BaseScrollBar::GetHideTimerForTest().
+
   // Methods to change the visibility of the scrollbar.
   void ShowScrollbar();
   void HideScrollbar();
@@ -100,6 +105,9 @@ class VIEWS_EXPORT CocoaScrollBar : public BaseScrollBar,
   // the scrollbar as the hiding animation.
   gfx::SlideAnimation thickness_animation_;
 
+  // The scroll offset from the last adjustment to the scrollbar.
+  int last_contents_scroll_offset_;
+
   // True when the scrollbar is expanded.
   bool is_expanded_;