cros: Add image_decoder service to //ash/manifest.json

The code in wallpaper_decoder.cc ([1]) uses the data decoder service
but forgot to update the manifest. Because of CL[2], it results in
crash.

[1] https://cs.chromium.org/chromium/src/ash/wallpaper/wallpaper_decoder.cc?type=cs&l=45
[2] https://chromium-review.googlesource.com/c/chromium/src/+/815995

Bug: 798922
Change-Id: I252a98f0e56821b3df4f46fb383cf2fee09f9ad7
Reviewed-on: https://chromium-review.googlesource.com/849431Reviewed-by: Greg Kerr <kerrnel@chromium.org>
Commit-Queue: Wenzhao (Colin) Zang <wzang@chromium.org>
Cr-Commit-Position: refs/heads/master@{#527066}

ash/manifest.json

@@ -50,7 +50,8 @@
         "catalog": [ "directory" ],
         "local_state": [ "pref_client" ],
         "ui": [ "display_dev", "window_manager", "video_detector" ],
-        "touch_hud": [ "mash:launchable" ]
+        "touch_hud": [ "mash:launchable" ],
+        "data_decoder": [ "image_decoder" ]
       }
     }
   }