Bug 1520088 - part5 - add pref to control whether blacklist can override the default setting. r=cpearce

Differential Revision: https://phabricator.services.mozilla.com/D16640

--HG--
extra : moz-landing-system : lando

dom/media/AutoplayPolicy.cpp

@@ -195,7 +195,8 @@ static bool IsAllowedToPlayInternal(const HTMLMediaElement& aElement) {
   }
 
   if (DefaultAutoplayBehaviour() == nsIAutoplay::ALLOWED &&
-      !IsSiteInAutoplayBlackList(approver)) {
+      !(IsSiteInAutoplayBlackList(approver) &&
+        StaticPrefs::MediaAutoplayBlackListOverrideDefault())) {
     AUTOPLAY_LOG(
         "Allow autoplay as global autoplay setting is allowing autoplay by "
         "default.");

modules/libpref/init/StaticPrefList.h

@@ -1604,6 +1604,12 @@ VARCACHE_PREF(
   RelaxedAtomicBool, true
 )
 
+VARCACHE_PREF(
+  "media.autoplay.blackList-override-default",
+   MediaAutoplayBlackListOverrideDefault,
+  RelaxedAtomicBool, true
+)
+
 //---------------------------------------------------------------------------
 // Network prefs
 //---------------------------------------------------------------------------