Support app window draggable areas in mash.

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 3daf0de42c5bea90ac2b409bcaf8f18e8c70ebcb..df6295e11e371c771307a6a8e0bcf0cd6f50874c 100644
--- a/ash/mus/property_util.cc
+++ b/ash/mus/property_util.cc
@@ -235,5 +235,12 @@ bool IsAlwaysOnTop(::mus::Window* window) {
              ::mus::mojom::WindowManager::kAlwaysOnTop_Property);
 }
 
+bool ShouldRemoveStandardFrame(::mus::Window* window) {
+  return window->HasSharedProperty(
+             ::mus::mojom::WindowManager::kRemoveStandardFrame_Property) &&
+         window->GetSharedProperty<bool>(
+             ::mus::mojom::WindowManager::kRemoveStandardFrame_Property);
+}
+
 }  // namespace mus
 }  // namespace ash