This is a backout of Bug 1347791 part 4; 49b533231388.
49b533231388 took the mediaBlocked attribute and stored it in session store,
and caused us to call browser.resumeMedia() or blockMedia() as appropriate
upon restore. We don't want to session restore whether tabs have had delay
playback start unblocked anymore, so remove the code that session stores
that attribute.
MozReview-Commit-ID: AkRVlufrUAK
35c33d46de