cheets: implement cros side of WallpaperManagerService.

components/arc/set_wallpaper_delegate.h

Index: components/arc/set_wallpaper_delegate.h
diff --git a/components/arc/set_wallpaper_delegate.h b/components/arc/set_wallpaper_delegate.h
index 28e5ed038c5248a219b2b3f78e1294c8d4a7cd5a..931cd0d764d21a4b290794f5025201ce717c1407 100644
--- a/components/arc/set_wallpaper_delegate.h
+++ b/components/arc/set_wallpaper_delegate.h
@@ -6,9 +6,10 @@
 #define COMPONENTS_ARC_SET_WALLPAPER_DELEGATE_H_
 
 #include <stdint.h>
-
 #include <vector>
 
+#include "base/callback_forward.h"
+
 namespace arc {
 
 // Delegate to allow setting the wallpaper.
@@ -17,7 +18,11 @@ class SetWallpaperDelegate {
   virtual ~SetWallpaperDelegate() = default;
 
   // Sets an image represented in JPEG format as the wallpaper.
-  virtual void SetWallpaper(std::vector<uint8_t> jpeg_data) = 0;
+  virtual void SetWallpaper(const std::vector<uint8_t>& jpeg_data) = 0;
+  // Gets current wallpaper encoded in JPEG format.
+  virtual void GetWallpaper(
+      const base::Callback<void(const std::vector<uint8_t>& image)>& callback)
+      const = 0;
 };
 
 }  // namespace arc