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..c854f26bfae2a8f2dc4c0e30145f94666cebf104 100644
--- a/ash/mus/BUILD.gn
+++ b/ash/mus/BUILD.gn
@@ -28,6 +28,8 @@ source_set("lib") {
     "stub_context_factory.h",
     "sysui_application.cc",
     "sysui_application.h",
+    "user_wallpaper_delegate_mus.cc",
+    "user_wallpaper_delegate_mus.h",
   ]
 
   deps = [
@@ -38,7 +40,9 @@ source_set("lib") {
     "//components/mus/public/cpp",
     "//components/mus/public/interfaces",
     "//components/user_manager",
+    "//components/wallpaper",
     "//device/bluetooth",
+    "//mash/public/interfaces",
     "//mash/shelf/public/interfaces",
     "//mash/wm/public/interfaces",
     "//mojo/common:common_base",
@@ -62,6 +66,7 @@ source_set("lib") {
     "//ui/strings",
     "//ui/views",
     "//ui/views/mus:for_mojo_application",
+    "//ui/wm",
   ]
 
   if (is_chromeos) {