Add flag to enable immersive fullscreen for v2 apps

ash/wm/window_state_delegate.h

Index: ash/wm/window_state_delegate.h
diff --git a/ash/wm/window_state_delegate.h b/ash/wm/window_state_delegate.h
index 7b9a8baa3d2cdd8341fc576fe14aef2efb2c90fe..53badcf415e9df751797aa507fe5326fa03b0a76 100644
--- a/ash/wm/window_state_delegate.h
+++ b/ash/wm/window_state_delegate.h
@@ -17,9 +17,12 @@ class ASH_EXPORT WindowStateDelegate {
   WindowStateDelegate();
   virtual ~WindowStateDelegate();
 
-  // Invoked when the user uses Shift+F4/F4 to toggle the window
-  // fullscreen state.  The caller (ash::wm::WindowState) falls backs
-  // to the default implementation if this returns false.
+  // Invoked when the user uses Shift+F4/F4 to toggle the window fullscreen
+  // state. If the window is not fullscreen and the window supports immersive
+  // fullscreen ToggleFullscreen() should put the window into immersive
+  // fullscreen instead of the default fullscreen type. The caller
+  // (ash::wm::WindowState) falls backs to the default implementation if this
+  // returns false.
   virtual bool ToggleFullscreen(WindowState* window_state);
 
  private: