Leave fullscreen mode in an app window when ESC key is pressed.

apps/shell_window.h

Index: apps/shell_window.h
diff --git a/apps/shell_window.h b/apps/shell_window.h
index c236a1d7354f0c6c3f5935b41870a48c817b22a0..bb841c3f2a16d843d2090269bd3765b57d3a6c3f 100644
--- a/apps/shell_window.h
+++ b/apps/shell_window.h
@@ -348,6 +348,10 @@ class ShellWindow : public content::NotificationObserver,
                               const gfx::Rect& initial_pos,
                               bool user_gesture,
                               bool* was_blocked) OVERRIDE;
+  virtual bool PreHandleKeyboardEvent(
+      content::WebContents* source,
+      const content::NativeWebKeyboardEvent& event,
+      bool* is_keyboard_shortcut) OVERRIDE;
   virtual void HandleKeyboardEvent(
       content::WebContents* source,
       const content::NativeWebKeyboardEvent& event) OVERRIDE;