Cleanup mojo Wallpaper interfaces for mash.

ash/public/interfaces/wallpaper.mojom

 // Copyright 2016 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 module ash.mojom;
 
 import "skia/public/interfaces/bitmap.mojom";
 
 // These values match wallpaper::WallpaperLayout.
 enum WallpaperLayout { CENTER, CENTER_CROPPED, STRETCH, TILE, };
 
 // Used by Chrome to set the wallpaper displayed by ash.
 interface WallpaperController {
+  // Set the wallpaper bitmap and layout used for the ash desktop background.
+  // A null or empty |wallpaper| bitmap is treated as a no-op.
+  // TODO(crbug.com/655875): Optimize ash wallpaper transport; avoid sending
+  // large bitmaps over Mojo; use shared memory like BitmapUploader, etc.
   SetWallpaper(skia.mojom.Bitmap? wallpaper, WallpaperLayout layout);
 };
 
 // Used by ash to trigger Chrome's wallpaper picker functionality.
 interface WallpaperManager {
+  // Open the wallpaper picker window.
   Open();
 };