Links in platform apps should open in the system default browser.

chrome/browser/ui/extensions/shell_window.h

Index: chrome/browser/ui/extensions/shell_window.h
diff --git a/chrome/browser/ui/extensions/shell_window.h b/chrome/browser/ui/extensions/shell_window.h
index 88bcfe11613fd6094d7ffa3e9b282241f467c45f..fe1ca68fd6769f7b74244fdecdea3c0225f8d636 100644
--- a/chrome/browser/ui/extensions/shell_window.h
+++ b/chrome/browser/ui/extensions/shell_window.h
@@ -97,6 +97,10 @@ class ShellWindow : public content::NotificationObserver,
   // Save the window position in the prefs.
   virtual void SaveWindowPosition();
 
+  // Set the WebContentsDelegate for external url in test.
+  static void SetExternalUrlControllerForTesting(
+      content::WebContentsDelegate* controller);
+
  protected:
   ShellWindow(Profile* profile,
               const extensions::Extension* extension);