Fills athena background always.

athena/util/fill_layout_manager.h

Index: athena/util/fill_layout_manager.h
diff --git a/athena/util/fill_layout_manager.h b/athena/util/fill_layout_manager.h
index 50e7df8ffeed3724b23e1ffb16628cd53598102a..32233e101916c8d2c894e9f08a11a0006d8f8efe 100644
--- a/athena/util/fill_layout_manager.h
+++ b/athena/util/fill_layout_manager.h
@@ -8,8 +8,19 @@
 #include "athena/athena_export.h"
 #include "ui/aura/layout_manager.h"
 
+namespace aura {
+template <typename T>
+struct WindowProperty;
+}
+
 namespace athena {
 
+// A property key for FillLayoutManager; FillLayoutManager always fills the
+// window when this key is set to true regardless of the window type. The
+// default value is false.
+ATHENA_EXPORT extern const aura::WindowProperty<bool>* const
+    kAlwaysFillWindowKey;
+
 class ATHENA_EXPORT FillLayoutManager : public aura::LayoutManager {
  public:
   explicit FillLayoutManager(aura::Window* container);