Add policy for fullscreen mode; disallow fullscreen in public sessions

apps/pref_names.cc

Index: apps/pref_names.cc
diff --git a/apps/pref_names.cc b/apps/pref_names.cc
index 02b652c316316e86bf52d45e9486dc44532c50c2..2ca3a4a617b53882e4d95561e4d99223720c5a97 100644
--- a/apps/pref_names.cc
+++ b/apps/pref_names.cc
@@ -8,6 +8,10 @@ namespace apps {
 
 namespace prefs {
 
+// A boolean that tracks whether apps are allowed to enter fullscreen mode.
+extern const char kAppFullscreenAllowed[] =
+    "apps.fullscreen.allowed";
+
 // A boolean that tracks whether the user has ever enabled the app launcher.
 const char kAppLauncherHasBeenEnabled[] =
     "apps.app_launcher.has_been_enabled";