Bug 1665355 - Don't throw on appcache.update() if the storage is disabled r=baku

To avoid applications breaking, this method shouldn't throw when called if the
pref is set to off.

To reproduce go to: https://webdbg.com/test/appcache/
Open devtools console
Type: appCache.update()
Make sure it doesn't throw

Differential Revision: https://phabricator.services.mozilla.com/D90390

dom/offline/nsDOMOfflineResourceList.cpp

@@ -26,6 +26,7 @@
 #include "mozilla/EventDispatcher.h"
 #include "mozilla/Preferences.h"
 #include "mozilla/BasePrincipal.h"
+#include "mozilla/StaticPrefs_browser.h"
 
 #include "nsXULAppAPI.h"
 #define IS_CHILD_PROCESS() (GeckoProcessType_Default != XRE_GetProcessType())
@@ -486,6 +487,11 @@ void nsDOMOfflineResourceList::Update(ErrorResult& aRv) {
     return;
   }
 
+  // If the storage is disabled this operation should be ignored.
+  if (!StaticPrefs::browser_cache_offline_storage_enable()) {
+    return;
+  }
+
   if (!nsContentUtils::OfflineAppAllowed(mDocumentURI)) {
     aRv.Throw(NS_ERROR_DOM_SECURITY_ERR);
     return;