Bug 1445556 - Remove media.cubeb.sandbox from MediaPrefs. r=cpearce

Like the other media.cubeb.* prefs, it doesn't need to be a VarCache pref.

MozReview-Commit-ID: A2L8Tf3GAt

--HG--
extra : rebase_source : 86791123613c2f21e4a771c99cb079125495dec3

dom/media/MediaPrefs.h

@@ -194,8 +194,6 @@ private:
   // resume background video decoding when the cursor is hovering over the tab.
   DECL_MEDIA_PREF("media.resume-bkgnd-video-on-tabhover",     ResumeVideoDecodingOnTabHover, bool, false);
 
-  // Enable sandboxing support for cubeb
-  DECL_MEDIA_PREF("media.cubeb.sandbox",                      CubebSandbox, bool, false);
   DECL_MEDIA_PREF("media.videocontrols.lock-video-orientation",  VideoOrientationLockEnabled, bool, false);
 
   // Media Seamless Looping