Support the ignored_by_shelf flag for Mus windows.

components/mus/public/interfaces/window_manager.mojom

Index: components/mus/public/interfaces/window_manager.mojom
diff --git a/components/mus/public/interfaces/window_manager.mojom b/components/mus/public/interfaces/window_manager.mojom
index 20e0c74f761a3077ea328afda4d5588f8787a0f2..558847161710bbc908488ed8deff08848cf5eb84 100644
--- a/components/mus/public/interfaces/window_manager.mojom
+++ b/components/mus/public/interfaces/window_manager.mojom
@@ -40,6 +40,8 @@ interface WindowManager {
   const string kWindowType_Property = "prop:window-type";
   // The window's title. Type: mojom::String
   const string kWindowTitle_Property = "prop:window-title";
+  // A flag controlling the window's presence on the mash shelf. Type: bool
+  const string kWindowIgnoredByShelf_Property = "prop:window-ignored-by-shelf";
   // The application ID (eg. 'mojo:foo'). Type: mojom::String
   const string kAppID_Property = "prop:app-id";