Make the lifetime of wallpaper manager more clear.

chrome/browser/chromeos/login/users/wallpaper/wallpaper_manager_unittest.cc

Index: chrome/browser/chromeos/login/users/wallpaper/wallpaper_manager_unittest.cc
diff --git a/chrome/browser/chromeos/login/users/wallpaper/wallpaper_manager_unittest.cc b/chrome/browser/chromeos/login/users/wallpaper/wallpaper_manager_unittest.cc
index b6543f5923de4e6df897bb71d6bf27f34dc35767..9c9f6154f958db9ac4bb7739d02cfb3455b0015b 100644
--- a/chrome/browser/chromeos/login/users/wallpaper/wallpaper_manager_unittest.cc
+++ b/chrome/browser/chromeos/login/users/wallpaper/wallpaper_manager_unittest.cc
@@ -47,7 +47,15 @@ class WallpaperManagerCacheTest : public test::AshTestBase {
 
   FakeChromeUserManager* fake_user_manager() { return fake_user_manager_; }
 
-  void SetUp() override { test::AshTestBase::SetUp(); }
+  void SetUp() override {
+    test::AshTestBase::SetUp();
+    WallpaperManager::Initialize();
+  }
+
+  void TearDown() override {
+    WallpaperManager::Shutdown();
+    test::AshTestBase::TearDown();
+  }
 
   // Creates a test image of size 1x1.
   gfx::ImageSkia CreateTestImage(SkColor color) {