This patch replaces the usage of sNextTabParent pointer to store the next
PBrowser parent actor to be used by the next frame loader with the
following information:
* In the case where the content JS has requested a new tab, the ID of the
next TabParent will be stored on the <xul:browser> element.
* In the case where the content JS has requested a new window, the ID of
the next TabParent will be stored on the created nsXULWindow.
