Remove remainder of USE_ATHENA ifdefs

chrome/browser/fullscreen_chromeos.cc

 // Copyright (c) 2013 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 "chrome/browser/fullscreen.h"
 
 #include "ash/root_window_controller.h"
 
 bool IsFullScreenMode() {
-#if defined(USE_ATHENA)
-  // crbug.com/408755
-  return false;
-#endif
   // TODO(oshima): Fullscreen is per display state. Investigate
   // and fix if necessary.
   ash::RootWindowController* controller =
       ash::RootWindowController::ForTargetRootWindow();
   return controller && controller->GetWindowForFullscreenMode();
 }