fix 'set but not used' GCC warning

ui/views/controls/menu/menu_controller.cc

 // 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.
 
 #include "ui/views/controls/menu/menu_controller.h"
 
 #include "base/i18n/case_conversion.h"
 #include "base/i18n/rtl.h"
 #include "base/macros.h"
 #include "base/strings/utf_string_conversions.h"
@@ skipping 2251 matching lines @@
 
   MenuMessageLoop::RepostEventToWindow(event, window, screen_loc);
 }
 
 void MenuController::RepostEventAndCancel(SubmenuView* source,
                                           const ui::LocatedEvent* event) {
   // Cancel can lead to the deletion |source| so we save the view and window to
   // be used when reposting the event.
   gfx::Point screen_loc(event->location());
   View::ConvertPointToScreen(source->GetScrollViewContainer(), &screen_loc);
+
+#if defined(OS_WIN) || defined(OS_CHROMEOS)
   gfx::NativeView native_view = source->GetWidget()->GetNativeView();
   gfx::NativeWindow window = nullptr;
   if (native_view) {
     gfx::Screen* screen = gfx::Screen::GetScreenFor(native_view);
     window = screen->GetWindowAtScreenPoint(screen_loc);
   }
+#endif
 
 #if defined(OS_WIN)
   // We're going to close and we own the event capture. We need to repost the
   // event, otherwise the window the user clicked on won't get the event.
   RepostEvent(source, event, screen_loc, native_view, window);
 #endif
 
   // Determine target to see if a complete or partial close of the menu should
   // occur.
   ExitType exit_type = EXIT_ALL;
@@ skipping 280 matching lines @@
   }
 }
 
 gfx::Screen* MenuController::GetScreen() {
   Widget* root = owner_ ? owner_->GetTopLevelWidget() : NULL;
   return root ? gfx::Screen::GetScreenFor(root->GetNativeView())
               : gfx::Screen::GetNativeScreen();
 }
 
 }  // namespace views