There are four call sites of FirePageHideNotification(). Two of them are
handled in this patch. The other two will be taken care of in the subsequent
patches in this patch series respectively.
Without this fix, the test case in this patch causes assertions when
cycle collection happens.
MozReview-Commit-ID: 6GSxjdfXGcY
--HG--
extra : rebase_source : 998b95ae90a1ad80dc177d5eecf1a592ba375116
Hiroyuki Ikezoe