Bug 1448222 - Remove MediaPrefs. r=jya

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

widget/android/AndroidBridge.cpp

@@ -48,7 +48,6 @@
 #include "mozilla/dom/ContentChild.h"
 #include "nsIObserverService.h"
 #include "nsISupportsPrimitives.h"
-#include "MediaPrefs.h"
 #include "WidgetUtils.h"
 
 #include "FennecJNIWrappers.h"
@@ -130,8 +129,6 @@ AndroidBridge::ConstructBridge()
 
     MOZ_ASSERT(!sBridge);
     sBridge = new AndroidBridge();
-
-    MediaPrefs::GetSingleton();
 }
 
 void