start/stop the source of the capturer when 1st audiotrack/last audiotrack is added/removed

content/renderer/media/webaudio_capturer_source.cc

 // Copyright (c) 2012 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/webaudio_capturer_source.h"
 
 #include "base/logging.h"
 #include "content/renderer/media/webrtc_audio_capturer.h"
 
 using media::AudioBus;
@@ skipping 19 matching lines @@
 
 void WebAudioCapturerSource::setFormat(
     size_t number_of_channels, float sample_rate) {
   DVLOG(1) << "WebAudioCapturerSource::setFormat(sample_rate="
            << sample_rate << ")";
   if (number_of_channels <= 2) {
     set_format_channels_ = number_of_channels;
     ChannelLayout channel_layout =
         number_of_channels == 1 ? CHANNEL_LAYOUT_MONO : CHANNEL_LAYOUT_STEREO;
     capturer_->SetCapturerSource(this, channel_layout, sample_rate);
-    capturer_->Start();
   } else {
     // TODO(crogers): Handle more than just the mono and stereo cases.
     LOG(WARNING) << "WebAudioCapturerSource::setFormat() : unhandled format.";
   }
 }
 
 void WebAudioCapturerSource::Initialize(
     const media::AudioParameters& params,
     media::AudioCapturerSource::CaptureCallback* callback,
     int session_id) {
@@ skipping 44 matching lines @@
 
   fifo_->Push(wrapper_bus_.get());
   int capture_frames = params_.frames_per_buffer();
   while (fifo_->frames() >= capture_frames) {
     fifo_->Consume(capture_bus_.get(), 0, capture_frames);
     callback_->Capture(capture_bus_.get(), 0, 1.0);
   }
 }
 
 }  // namespace content