Revert of Remove ignored return code from modules.

webrtc/modules/audio_conference_mixer/test/audio_conference_mixer_unittest.cc

 /*
  *  Copyright (c) 2015 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 127 matching lines @@
   }
 
   // Last participant gives audio frame with passive VAD, although it has the
   // largest energy.
   participants[kParticipants - 1].fake_frame()->vad_activity_ =
       AudioFrame::kVadPassive;
 
   EXPECT_CALL(output_receiver, NewMixedAudio(_, _, _, _))
       .Times(AtLeast(1));
 
-  mixer->Process();
+  EXPECT_EQ(0, mixer->Process());
 
   for (int i = 0; i < kParticipants; ++i) {
     bool is_mixed = participants[i].IsMixed();
     if (i == kParticipants - 1 || i < kParticipants - 1 -
         AudioConferenceMixer::kMaximumAmountOfMixedParticipants) {
       EXPECT_FALSE(is_mixed) << "Mixing status of Participant #"
                              << i << " wrong.";
     } else {
       EXPECT_TRUE(is_mixed) << "Mixing status of Participant #"
                             << i << " wrong.";
     }
   }
 
   EXPECT_EQ(0, mixer->UnRegisterMixedStreamCallback());
 }
 
 }  // namespace webrtc