reland of https://codereview.chromium.org/11364053/ w/o removing the per-rootwindow stacking client

chrome/browser/ui/aura/chrome_browser_main_extra_parts_aura.cc

Index: chrome/browser/ui/aura/chrome_browser_main_extra_parts_aura.cc
diff --git a/chrome/browser/ui/aura/chrome_browser_main_extra_parts_aura.cc b/chrome/browser/ui/aura/chrome_browser_main_extra_parts_aura.cc
index 73b1d439fb452e32ef171cb8c18e12d3dcad1a76..52ecc6e96f1d6bf24de042f53c676a767fa360f2 100644
--- a/chrome/browser/ui/aura/chrome_browser_main_extra_parts_aura.cc
+++ b/chrome/browser/ui/aura/chrome_browser_main_extra_parts_aura.cc
@@ -6,6 +6,7 @@
 
 #include "chrome/browser/chrome_browser_main.h"
 #include "chrome/browser/toolkit_extra_parts.h"
+#include "chrome/browser/ui/aura/stacking_client_aura.h"
 #include "ui/aura/desktop/desktop_screen.h"
 #include "ui/aura/desktop/desktop_stacking_client.h"
 #include "ui/aura/env.h"
@@ -16,6 +17,7 @@
 #if defined(OS_LINUX)
 #include "chrome/browser/ui/libgtk2ui/gtk2_ui.h"
 #include "ui/base/linux_ui.h"
+#else
 #endif
 
 #if defined(USE_ASH)
@@ -37,7 +39,8 @@ void ChromeBrowserMainExtraPartsAura::PreProfileInit() {
     gfx::Screen::SetScreenInstance(
         gfx::SCREEN_TYPE_NATIVE, aura::CreateDesktopScreen());
     aura::Env::GetInstance()->SetDisplayManager(new aura::SingleDisplayManager);
-    stacking_client_.reset(new aura::DesktopStackingClient);
+    stacking_client_.reset(new StackingClientAura);
+    aura::client::SetStackingClient(stacking_client_.get());
   }
 #endif