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_unittest.cc

@@ -169,7 +169,8 @@ TEST_F(BitrateControllerTest, OneBitrateObserverOneRtcpObserver) {
   EXPECT_EQ(300000, bitrate_observer_.last_bitrate_);
 
   // Test that a low delay-based estimate limits the combined estimate.
-  controller_->OnReceiveBitrateChanged({0}, 280000);
+  webrtc::DelayBasedBwe::Result result(false, 280000);
+  controller_->OnDelayBasedBweResult(result);
   EXPECT_EQ(280000, bitrate_observer_.last_bitrate_);
 
   // Test that a low REMB limits the combined estimate.