Fix flaky ViEEncoder unit test.

The flaky test was introduced in ad9010c9836, and is essentially a race
where the ViE Encoder has already configured the quality scaler on the
encoder thread before we've updated the ScalingSettings. This CL adds
a forced reconfiguration of the quality scaler to avoid this issue.

BUG=None
TBR=sprang@webrtc.org

Review-Url: https://codereview.webrtc.org/2695873004
Cr-Commit-Position: refs/heads/master@{#16612}
diff --git a/webrtc/video/vie_encoder_unittest.cc b/webrtc/video/vie_encoder_unittest.cc
index 3ecfec8..380c4fa 100644
--- a/webrtc/video/vie_encoder_unittest.cc
+++ b/webrtc/video/vie_encoder_unittest.cc
@@ -1186,6 +1186,9 @@
   int frame_height = 360;
   fake_encoder_.SetQualityScaling(false);
   vie_encoder_->OnBitrateUpdated(kLowTargetBitrateBps, 0, 0);
+  // Force quality scaler reconfiguration by resetting the source.
+  vie_encoder_->SetSource(&video_source_,
+                          VideoSendStream::DegradationPreference::kBalanced);
 
   video_source_.IncomingCapturedFrame(
       CreateFrame(1, frame_width, frame_height));