Preload default wallpaper.

ash/desktop_background/desktop_background_controller.cc

Index: ash/desktop_background/desktop_background_controller.cc
diff --git a/ash/desktop_background/desktop_background_controller.cc b/ash/desktop_background/desktop_background_controller.cc
index 022ca0ba0c796d47753b6536e39bc8dac85d94e0..d311636706d29d531166d8f8d3af586faa32ecbe 100644
--- a/ash/desktop_background/desktop_background_controller.cc
+++ b/ash/desktop_background/desktop_background_controller.cc
@@ -134,6 +134,18 @@ void DesktopBackgroundController::OnRootWindowAdded(
   }
 }
 
+void DesktopBackgroundController::TriggerCacheDefaultWallpaper(int index) {
+  if (index == ash::GetInvalidWallpaperIndex() ||
+      index == ash::GetSolidColorIndex())
+    return;
+  scoped_refptr<WallpaperOperation> wallpaper_op =
+      new WallpaperOperation(index);
+  base::WorkerPool::PostTask(
+      FROM_HERE,
+      base::Bind(&WallpaperOperation::Run, wallpaper_op),
+      true);
+}
+
 void DesktopBackgroundController::SetDefaultWallpaper(int index) {
   // We should not change background when index is invalid. For instance, at
   // login screen or stub_user login.