Make the lifetime of wallpaper manager more clear.

chrome/browser/chromeos/login/users/user_manager_unittest.cc

Index: chrome/browser/chromeos/login/users/user_manager_unittest.cc
diff --git a/chrome/browser/chromeos/login/users/user_manager_unittest.cc b/chrome/browser/chromeos/login/users/user_manager_unittest.cc
index 85e718b493a2b2b7761394ce956f85eaa70bc8dc..3765efa4651b545664701ac8c717fd3691647004 100644
--- a/chrome/browser/chromeos/login/users/user_manager_unittest.cc
+++ b/chrome/browser/chromeos/login/users/user_manager_unittest.cc
@@ -15,6 +15,7 @@
 #include "chrome/browser/browser_process.h"
 #include "chrome/browser/chromeos/login/users/chrome_user_manager_impl.h"
 #include "chrome/browser/chromeos/login/users/scoped_user_manager_enabler.h"
+#include "chrome/browser/chromeos/login/users/wallpaper/wallpaper_manager.h"
 #include "chrome/browser/chromeos/settings/cros_settings.h"
 #include "chrome/browser/chromeos/settings/device_settings_service.h"
 #include "chrome/browser/chromeos/settings/stub_cros_settings_provider.h"
@@ -82,6 +83,7 @@ class UserManagerTest : public testing::Test {
     chromeos::DBusThreadManager::Initialize();
 
     ResetUserManager();
+    WallpaperManager::Initialize();
   }
 
   void TearDown() override {
@@ -99,6 +101,7 @@ class UserManagerTest : public testing::Test {
 
     base::RunLoop().RunUntilIdle();
     chromeos::DBusThreadManager::Shutdown();
+    WallpaperManager::Shutdown();
   }
 
   ChromeUserManagerImpl* GetChromeUserManager() const {