Called on the UI thread before a new popup browser is created. The |browser| and |frame| values represent the source of the popup request. The |target_url| and |target_frame_name| values indicate where the popup browser should navigate and may be NULL if not specified with the request. The |target_disposition| value indicates where the user intended to open the popup (e.g. current tab, new tab, etc). The |user_gesture| value will be true (1) if the popup was opened via explicit user gesture (e.g.