Fix Memcheck:Uninitialized error caused by r309316.

content/renderer/media/speech_recognition_audio_sink_unittest.cc

Index: content/renderer/media/speech_recognition_audio_sink_unittest.cc
diff --git a/content/renderer/media/speech_recognition_audio_sink_unittest.cc b/content/renderer/media/speech_recognition_audio_sink_unittest.cc
index 2ae1d05246ffdfd86bf6acb07939bcfc26260ecf..12e8cc1825cc8d32dc3f0b58fa3d20b7e6459b40 100644
--- a/content/renderer/media/speech_recognition_audio_sink_unittest.cc
+++ b/content/renderer/media/speech_recognition_audio_sink_unittest.cc
@@ -226,6 +226,7 @@ class SpeechRecognitionAudioSinkTest : public testing::Test {
                        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;