Fix flaky ViEEncoder unit test.

webrtc/video/vie_encoder_unittest.cc

Index: webrtc/video/vie_encoder_unittest.cc
diff --git a/webrtc/video/vie_encoder_unittest.cc b/webrtc/video/vie_encoder_unittest.cc
index 3ecfec85b8ac0b0fec0e30d6135acd3f1ca5d225..380c4fa768edafdfb6bf6ad75b8db77fcbb86444 100644
--- a/webrtc/video/vie_encoder_unittest.cc
+++ b/webrtc/video/vie_encoder_unittest.cc
@@ -1186,6 +1186,9 @@ TEST_F(ViEEncoderTest, InitialFrameDropOffWhenEncoderDisabledScaling) {
   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));