ACM2/NetEq4 did not decode Opus in stereo

Two problems fixed in this CL:
- setting Opus decoder to stereo had no effect, and decoding always generated mono audio
- changing decoding setting from mono to stereo, or stereo to mono, for OPUS also had no effect (but required another change than the first one).

BUG=3082
R=henrik.lundin@webrtc.org, turaj@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/10389004

git-svn-id: http://webrtc.googlecode.com/svn/trunk@5754 4adac7df-926f-26a2-2b94-8c16560cd09d

webrtc/modules/audio_coding/main/test/opus_test.cc

@@ -333,6 +333,11 @@ void OpusTest::Run(TestPackStereo* channel, int channels, int bitrate,
 
     // Write stand-alone speech to file.
     out_file_standalone_.Write10MsData(out_audio, decoded_samples * channels);
+
+    // Number of channels should be the same for both stand-alone and
+    // ACM-decoding.
+    EXPECT_EQ(audio_frame.num_channels_, channels);
+
     decoded_samples = 0;
   }