Hook up Chrome's wallpaper picker for mash.

ash/mus/BUILD.gn

Index: ash/mus/BUILD.gn
diff --git a/ash/mus/BUILD.gn b/ash/mus/BUILD.gn
index da25a6e595f84e998d6d028189020d732b7ba3a6..7b5a9f9e7b5ef6e4046e879fd8d12fa84f24807c 100644
--- a/ash/mus/BUILD.gn
+++ b/ash/mus/BUILD.gn
@@ -28,16 +28,20 @@ source_set("lib") {
     "stub_context_factory.h",
     "sysui_application.cc",
     "sysui_application.h",
+    "user_wallpaper_delegate_mus.cc",
+    "user_wallpaper_delegate_mus.h",
   ]
 
   deps = [
     "//ash",
+    "//ash/mus/public/interfaces",
     "//cc",
     "//cc/surfaces",
     "//components/mus/common:mus_common",
     "//components/mus/public/cpp",
     "//components/mus/public/interfaces",
     "//components/user_manager",
+    "//components/wallpaper",
     "//device/bluetooth",
     "//mash/shelf/public/interfaces",
     "//mash/wm/public/interfaces",
@@ -62,6 +66,7 @@ source_set("lib") {
     "//ui/strings",
     "//ui/views",
     "//ui/views/mus:for_mojo_application",
+    "//ui/wm",
   ]
 
   if (is_chromeos) {