Bug 1536236 - Make sure mDNSPrefetch is always released on main thread r=valentin

This patch should make mDNSPrefetch to be released always on main thread.

Differential Revision: https://phabricator.services.mozilla.com/D24685

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

netwerk/protocol/http/nsHttpChannel.cpp

@@ -371,6 +371,7 @@ void nsHttpChannel::ReleaseMainThreadOnlyReferences() {
   arrayToRelease.AppendElement(mRedirectURI.forget());
   arrayToRelease.AppendElement(mRedirectChannel.forget());
   arrayToRelease.AppendElement(mPreflightChannel.forget());
+  arrayToRelease.AppendElement(mDNSPrefetch.forget());
 
   NS_DispatchToMainThread(new ProxyReleaseRunnable(std::move(arrayToRelease)));
 }
@@ -7805,7 +7806,6 @@ nsHttpChannel::OnStopRequest(nsIRequest *request, nsresult status) {
       mTransactionTimings.domainLookupStart = mDNSPrefetch->StartTimestamp();
       mTransactionTimings.domainLookupEnd = mDNSPrefetch->EndTimestamp();
     }
-    mDNSPrefetch = nullptr;
 
     // handle auth retry...
     if (authRetry) {