Fix race / crash in OnNetworkRouteChanged().

To achieve this some refactoring was done to make it possible to synchronize
access to the DelayBasedBwe in TransportFeedbackAdapter:
- The callback was removed from DelayBasedBwe, it now instead returns its
  result.
- TransportFeedbackAdapter was moved to modules/congestion_controller to avoid
  unnecessary dependencies.

Reenables previously disabled flaky test. Can no longer reproduce flakiness with gtest-parallel and asan/tsan builds.

BUG=webrtc:6427, webrtc:6422

Review-Url: https://codereview.webrtc.org/2366333003
Cr-Commit-Position: refs/heads/master@{#14430}

webrtc/modules/bitrate_controller/bitrate_controller_impl.h

@@ -61,10 +61,7 @@ class BitrateControllerImpl : public BitrateController {
                             uint8_t* fraction_loss,
                             int64_t* rtt) override;
 
-  // RemoteBitrateObserver overrides.
-  void OnReceiveBitrateChanged(const std::vector<uint32_t>& ssrcs,
-                               uint32_t bitrate_bps) override;
-  void OnProbeBitrate(uint32_t bitrate_bps) override;
+  void OnDelayBasedBweResult(const DelayBasedBwe::Result& result) override;
 
   int64_t TimeUntilNextProcess() override;
   void Process() override;