Bug 1466129 - Add default value to pref. r=johannh

MozReview-Commit-ID: 988Co3PD41V

--HG--
extra : rebase_source : fe5eeccf370c23525c72f427dcc65bb973b84b32

browser/components/preferences/in-content/privacy.js

@@ -1006,7 +1006,7 @@ var gPrivacyPane = {
    */
   updateAutoplayMediaControlsVisibility() {
     document.getElementById("autoplayMediaBox").hidden =
-      !Services.prefs.getBoolPref("media.autoplay.enabled.user-gestures-needed");
+      !Services.prefs.getBoolPref("media.autoplay.enabled.user-gestures-needed", false);
   },
 
   /**