Fix Memcheck:Uninitialized error caused by r309316.

content/renderer/media/speech_recognition_audio_sink_unittest.cc

 // Copyright 2014 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "content/renderer/media/speech_recognition_audio_sink.h"
 
 #include "base/bind.h"
 #include "base/strings/utf_string_conversions.h"
 #include "content/renderer/media/media_stream_audio_source.h"
 #include "content/renderer/media/mock_media_constraint_factory.h"
@@ skipping 208 matching lines @@
                          kInputBitsPerSample,
                          input_frames_per_buffer);
     sink_params_.Reset(kOutputFormat,
                        kOutputChannelLayout,
                        kOutputChannels,
                        output_sample_rate,
                        kOutputBitsPerSample,
                        output_frames_per_buffer);
     source_bus_ =
         media::AudioBus::Create(kInputChannels, input_frames_per_buffer);
+    source_bus_->Zero();
     first_frame_capture_time_ = base::TimeTicks::Now();
     sample_frames_captured_ = 0;
 
     // Prepare the track and audio source.
     blink::WebMediaStreamTrack blink_track;
     PrepareBlinkTrackOfType(MEDIA_DEVICE_AUDIO_CAPTURE, &blink_track);
 
     // Get the native track from the blink track and initialize.
     native_track_ =
         static_cast<WebRtcLocalAudioTrack*>(blink_track.extraData());
@@ skipping 281 matching lines @@
   const uint32 buffers_per_notification = Initialize(44100, 441, 16000, 1600);
   AssertConsumedBuffers(0U);
   CaptureAudioAndAssertConsumedBuffers(buffers_per_notification, 1U);
   EXPECT_CALL(*this, StoppedCallback()).Times(1);
 
   native_track()->Stop();
   CaptureAudioAndAssertConsumedBuffers(buffers_per_notification, 1U);
 }
 
 }  // namespace content