This patch converts all the prefs in MediaPrefs to the new StaticPrefs system.

Note that the "media.wmf.skip-blacklist" pref was present in both MediaPrefs
and gfxPrefs. The copy in MediaPrefs was never used; this explains why this
patch does not add an entry for it to StaticPrefList.h.

Note also that the patch removes themedia.rust.mp4parser pref, because it's
unused.

MozReview-Commit-ID: IfHP37NbIjY

--HG--
extra : rebase_source : df84ea813b7c366d7be663c696891325610149c8