Use overlay scrollbars for the cros system menu.

ash/common/system/tray/fixed_sized_scroll_view.h

Index: ash/common/system/tray/fixed_sized_scroll_view.h
diff --git a/ash/common/system/tray/fixed_sized_scroll_view.h b/ash/common/system/tray/fixed_sized_scroll_view.h
index 4cccef45001c0cbd6dd6239a8088f7504ed54dc1..d45c479f5b9ece0671621d051fd38dc991350fd1 100644
--- a/ash/common/system/tray/fixed_sized_scroll_view.h
+++ b/ash/common/system/tray/fixed_sized_scroll_view.h
@@ -11,6 +11,8 @@
 namespace ash {
 
 // A custom scroll-view that has a specified dimension.
+// TODO(estade): can we get rid of this class? In MD, it hardly differs in
+// behavior from ScrollView.
 class FixedSizedScrollView : public views::ScrollView {
  public:
   FixedSizedScrollView();
@@ -22,7 +24,7 @@ class FixedSizedScrollView : public views::ScrollView {
   // PreferredSizeChanged()).
   void SetFixedSize(const gfx::Size& size);
 
-  void set_fixed_size(const gfx::Size& size) { fixed_size_ = size; }
+  void set_fixed_size(const gfx::Size& size);
 
   // Overridden from views::View:
   gfx::Size GetPreferredSize() const override;