Bug 1734573 - Call errorCheck after onStartRequest, r=mixedpuppy

Differential Revision: https://phabricator.services.mozilla.com/D132358

toolkit/components/extensions/webrequest/WebRequest.jsm

@@ -744,11 +744,12 @@ HttpObserverManager = {
       lastActivity &&
       lastActivity !== this.GOOD_LAST_ACTIVITY
     ) {
-      // Make a trip through the event loop to make sure errors have a
-      // chance to be processed before we fall back to a generic error
-      // string.
-      Services.tm.dispatchToMainThread(() => {
-        channel.errorCheck();
+      // Since the channel's security info is assigned in onStartRequest and
+      // errorCheck is called in ChannelWrapper::onStartRequest, we should check
+      // the errorString after onStartRequest to make sure errors have a chance
+      // to be processed before we fall back to a generic error string.
+      let onStart = function() {
+        channel.removeEventListener("start", onStart);
         if (!channel.errorString) {
           this.runChannelListener(channel, "onErrorOccurred", {
             error:
@@ -756,7 +757,8 @@ HttpObserverManager = {
               `NS_ERROR_NET_UNKNOWN_${lastActivity}`,
           });
         }
-      });
+      };
+      channel.addEventListener("start", onStart);
     } else if (
       lastActivity !== this.GOOD_LAST_ACTIVITY &&
       lastActivity !==