Support app window draggable areas in mash.

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 0b31c9541854cd37323469e87da9abd11a674488..571ca354d316ef500440d07c5c280b7472672991 100644
--- a/components/mus/public/interfaces/window_manager.mojom
+++ b/components/mus/public/interfaces/window_manager.mojom
@@ -50,6 +50,9 @@ interface WindowManager {
   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";
+  // Specifies that the system default caption and icon should not be rendered,
+  // and the client area should be equivalent to the window area. Type: bool
+  const string kRemoveStandardFrame_Property = "prop:remove-standard-frame";
 
   // When the WindowManager completes a request it must call back to
   // WindowManagerClient::WmResponse().