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..3784d1f668b59e6906c405325673c7db731b07b1 100644
--- a/ash/accelerators/accelerator_commands.h
+++ b/ash/accelerators/accelerator_commands.h
@@ -21,6 +21,11 @@ ASH_EXPORT bool ToggleMinimized();
 // fullscreen mode.
 ASH_EXPORT void ToggleMaximized();
 
+// Toggles the fullscreen state. If |WindowState::maximize_when_fullscreen()|
+// returns true, it will toggle maximized mode instead.  If the window is
+// already in maximized, it exits maximized mode.
+ASH_EXPORT void ToggleFullscreen();
+
 }  // namespace accelerators
 }  // namespace ash