Cleanup mojo Wallpaper interfaces for mash.

ash/public/interfaces/wallpaper.mojom

Index: ash/public/interfaces/wallpaper.mojom
diff --git a/ash/public/interfaces/wallpaper.mojom b/ash/public/interfaces/wallpaper.mojom
index fd04d84fa62fe584df94a00be35bd3991277ae8c..e03b28c89d34cdac3e7abca688cd8c4673b71698 100644
--- a/ash/public/interfaces/wallpaper.mojom
+++ b/ash/public/interfaces/wallpaper.mojom
@@ -11,10 +11,15 @@ 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();
 };