Add CHECK in Job::DoCreateStream().

Bug: 819101
Change-Id: Ic5070b5c26313a6b4c991abfc83228f9ef12f99b
Reviewed-on: https://chromium-review.googlesource.com/990532Reviewed-by: Ryan Hamilton <rch@chromium.org>
Commit-Queue: Bence Béky <bnc@chromium.org>
Cr-Commit-Position: refs/heads/master@{#547925}

net/http/http_stream_factory_impl_job.cc

@@ -1234,6 +1234,12 @@ int HttpStreamFactoryImpl::Job::DoCreateStream() {
   // It is possible that a pushed stream has been opened by a server since last
   // time Job checked above.
   if (!existing_spdy_session_) {
+    // WebSocket over HTTP/2 is only allowed to use existing HTTP/2 connections.
+    // Therefore |using_spdy_| could not have been set unless a connection had
+    // already been found.
+    // TODO(bnc): Change to DCHECK once https://crbug.com/819101 is fixed.
+    CHECK(!try_websocket_over_http2_);
+
     session_->spdy_session_pool()->push_promise_index()->ClaimPushedStream(
         spdy_session_key_, origin_url_, request_info_, &existing_spdy_session_,
         &pushed_stream_id_);