Hook up Chrome's wallpaper picker for mash.

ash/mus/public/interfaces/wallpaper.mojom

Index: ash/mus/public/interfaces/wallpaper.mojom
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+// 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.mus.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_sysui.
+interface WallpaperController {
+  SetWallpaper(skia.mojom.Bitmap wallpaper, WallpaperLayout layout);
+};
+
+// Used by ash_sysui to trigger Chrome's wallpaper picker functionality.
+interface WallpaperManager {
+  Open();
+};