Fix flaky ViEEncoder unit test.

webrtc/video/vie_encoder_unittest.cc

 /*
  *  Copyright (c) 2016 The WebRTC project authors. All Rights Reserved.
  *
  *  Use of this source code is governed by a BSD-style license
  *  that can be found in the LICENSE file in the root of the source
  *  tree. An additional intellectual property rights grant can be found
  *  in the file PATENTS.  All contributing project authors may
  *  be found in the AUTHORS file in the root of the source tree.
  */
 
@@ skipping 1168 matching lines @@
   sink_.WaitForEncodedFrame(1);
 
   vie_encoder_->Stop();
 }
 
 TEST_F(ViEEncoderTest, InitialFrameDropOffWhenEncoderDisabledScaling) {
   int frame_width = 640;
   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));
   // Frame should not be dropped, even if it's too large.
   sink_.WaitForEncodedFrame(1);
 
   vie_encoder_->Stop();
   fake_encoder_.SetQualityScaling(true);
 }
 
@@ skipping 19 matching lines @@
 
   // Trigger CPU normal use, return to original resoluton;
   vie_encoder_->TriggerCpuNormalUsage();
   video_source_.IncomingCapturedFrame(
       CreateFrame(3, kFrameWidth, kFrameHeight));
   sink_.WaitForEncodedFrame(kFrameWidth, kFrameHeight);
 
   vie_encoder_->Stop();
 }
 }  // namespace webrtc