Support the ignored_by_shelf flag for Mus windows.

mash/wm/property_util.cc

Index: mash/wm/property_util.cc
diff --git a/mash/wm/property_util.cc b/mash/wm/property_util.cc
index e8355fd57e4a665406dcb3d3cc9944a7a1a06937..13c29f051dbdd720fe720a450307529932cc616e 100644
--- a/mash/wm/property_util.cc
+++ b/mash/wm/property_util.cc
@@ -162,5 +162,12 @@ base::string16 GetAppID(const mus::Window* window) {
       mus::mojom::WindowManager::kAppID_Property);
 }
 
+bool GetWindowIgnoredByShelf(mus::Window* window) {
+  return window->HasSharedProperty(
+             mus::mojom::WindowManager::kWindowIgnoredByShelf_Property) &&
+         window->GetSharedProperty<bool>(
+             mus::mojom::WindowManager::kWindowIgnoredByShelf_Property);
+}
+
 }  // namespace wm
 }  // namespace mash