mash: Support ShelfWindowWatcher via ShelfItem properties.

ash/mus/property_util.cc

Index: ash/mus/property_util.cc
diff --git a/ash/mus/property_util.cc b/ash/mus/property_util.cc
index 13c3359408c9e76c8b77a9f9a771dd8fa1a3ceb2..801a44071d2043fc5e193de042b1733178e1dfb0 100644
--- a/ash/mus/property_util.cc
+++ b/ash/mus/property_util.cc
@@ -175,6 +175,11 @@ mojom::AshWindowType GetAshWindowType(const ui::Window* window) {
       window->GetSharedProperty<int32_t>(mojom::kAshWindowType_Property));
 }
 
+void SetWindowTitle(ui::Window* window, base::string16 title) {
+  window->SetSharedProperty<base::string16>(
+      ui::mojom::WindowManager::kWindowTitle_Property, title);
+}
+
 base::string16 GetWindowTitle(const ui::Window* window) {
   if (!window->HasSharedProperty(
           ui::mojom::WindowManager::kWindowTitle_Property)) {