mash: Have chrome set itself as the wallpaper picker.

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 e03b28c89d34cdac3e7abca688cd8c4673b71698..7215c319a52a0257f914452335ed6fd9f2d74eae 100644
--- a/ash/public/interfaces/wallpaper.mojom
+++ b/ash/public/interfaces/wallpaper.mojom
@@ -11,6 +11,9 @@ enum WallpaperLayout { CENTER, CENTER_CROPPED, STRETCH, TILE, };
 
 // Used by Chrome to set the wallpaper displayed by ash.
 interface WallpaperController {
+  // Set the wallpaper picker interface, to let ash trigger Chrome's picker.
+  SetWallpaperPicker(WallpaperPicker picker);
+
   // 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
@@ -19,7 +22,7 @@ interface WallpaperController {
 };
 
 // Used by ash to trigger Chrome's wallpaper picker functionality.
-interface WallpaperManager {
+interface WallpaperPicker {
   // Open the wallpaper picker window.
   Open();
 };