Bug 1381311 - change main thread check to STS thread check. r=mayhemer

In Bug 1338493 I moved the operation thread of HttpBackgroundChannelChild from main thread to STS thread.
All the thread assertion shoudl be updated.

MozReview-Commit-ID: 307jBR48aYc

--HG--
extra : rebase_source : e9bc56d0c70a3a283cda07669e79a59ccf651b33

netwerk/protocol/http/HttpBackgroundChannelChild.cpp

@@ -452,7 +452,7 @@ HttpBackgroundChannelChild::ActorDestroy(ActorDestroyReason aWhy)
     RefPtr<HttpBackgroundChannelChild> self = this;
     mQueuedRunnables.AppendElement(NS_NewRunnableFunction(
       "HttpBackgroundChannelChild::ActorDestroy", [self]() {
-        MOZ_ASSERT(NS_IsMainThread());
+        MOZ_ASSERT(OnSocketThread());
         RefPtr<HttpChannelChild> channelChild = self->mChannelChild.forget();
 
         if (channelChild) {