Introduce WindowStateDelegate::ToggleFullscreen

ash/accelerators/accelerator_commands.h

Index: ash/accelerators/accelerator_commands.h
diff --git a/ash/accelerators/accelerator_commands.h b/ash/accelerators/accelerator_commands.h
index 36006e7ebede569ef092457fd92d14bdf1fb86a0..df576ba975f02e96636e3db622a1f302ac96126d 100644
--- a/ash/accelerators/accelerator_commands.h
+++ b/ash/accelerators/accelerator_commands.h
@@ -21,6 +21,10 @@ ASH_EXPORT bool ToggleMinimized();
 // fullscreen mode.
 ASH_EXPORT void ToggleMaximized();
 
+// Toggles the fullscreen state. The behavior can be overridden
+// by WindowStateDelegate::ToggleFullscreen().
+ASH_EXPORT void ToggleFullscreen();
+
 }  // namespace accelerators
 }  // namespace ash