Fix broken BidirectionalStreamTest in Cronet on iOS.

Cq-Include-Trybots: master.tryserver.chromium.android:android_cronet_tester;master.tryserver.chromium.mac:ios-simulator-cronet
Bug: 836823, 829429
Change-Id: Id69a596b5d260ceec0794c6c6478ad01ce61a0da
Reviewed-on: https://chromium-review.googlesource.com/1028630Reviewed-by: Ryan Hamilton <rch@chromium.org>
Commit-Queue: Misha Efimov <mef@chromium.org>
Cr-Commit-Position: refs/heads/master@{#553832}

net/http/http_stream_factory.h

@@ -96,7 +96,9 @@ class NET_EXPORT HttpStreamFactory {
   // Request a BidirectionalStreamImpl.
   // Will call delegate->OnBidirectionalStreamImplReady on successful
   // completion.
-  std::unique_ptr<HttpStreamRequest> RequestBidirectionalStreamImpl(
+  // TODO(https://crbug.com/836823): This method is virtual to avoid cronet_test
+  // failure on iOS that is caused by Network Thread TLS getting the wrong slot.
+  virtual std::unique_ptr<HttpStreamRequest> RequestBidirectionalStreamImpl(
       const HttpRequestInfo& info,
       RequestPriority priority,
       const SSLConfig& server_ssl_config,