Use overlay scrollbars for the cros system menu.

ash/common/system/chromeos/ime_menu/ime_menu_tray.cc

 // 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.
 
 #include "ash/common/system/chromeos/ime_menu/ime_menu_tray.h"
 
 #include "ash/common/accessibility_delegate.h"
 #include "ash/common/ash_constants.h"
 #include "ash/common/material_design/material_design_controller.h"
 #include "ash/common/session/session_state_delegate.h"
@@ skipping 346 matching lines @@
         views::CreateEmptyBorder(0, 0, kMenuSeparatorVerticalPadding, 0));
     bubble_view->AddChildView(separator);
   }
 
   // Adds IME list to the bubble.
   ime_list_view_ =
       new ImeListView(nullptr, false, ImeListView::SHOW_SINGLE_IME);
 
   uint32_t current_height = ime_list_view_->scroll_content()->height();
   const gfx::Range height_range = GetImeListViewRange();
-  if (current_height > height_range.end()) {
+
+  if (MaterialDesignController::IsSystemTrayMenuMaterial()) {
+    ime_list_view_->scroller()->ClipHeightTo(height_range.start(),
+                                             height_range.end());
+  } else if (current_height > height_range.end()) {
     ime_list_view_->scroller()->SetFixedSize(
         gfx::Size(kTrayPopupMaxWidth, height_range.end()));
-  } else if (MaterialDesignController::IsSystemTrayMenuMaterial() &&
-             current_height < height_range.start()) {
+  } else if (current_height < height_range.start()) {
     ime_list_view_->scroller()->SetFixedSize(
         gfx::Size(kTrayPopupMaxWidth, height_range.start()));
   }
   bubble_view->AddChildView(ime_list_view_);
 
   // The bottom view that contains buttons are not supported in login/lock
   // screen.
   if (!IsInLoginOrLockScreen()) {
     if (ShouldShowEmojiHandwritingVoiceButtons()) {
       bubble_view->AddChildView(
@@ skipping 198 matching lines @@
   WmShell::Get()->system_tray_delegate()->GetCurrentIME(&current_ime_);
 
   // Updates the tray label based on the current input method.
   if (current_ime_.third_party)
     label_->SetText(current_ime_.short_name + base::UTF8ToUTF16("*"));
   else
     label_->SetText(current_ime_.short_name);
 }
 
 }  // namespace ash