Fix getUserMedia so that failure is reported for invalid audio sources.

media/audio/audio_input_device.cc

Index: media/audio/audio_input_device.cc
diff --git a/media/audio/audio_input_device.cc b/media/audio/audio_input_device.cc
index 9ebe4805ae010a2b514a95bc819bd47a9754e585..a6f93a3ab6f5117a11d8050859f4c6167419b6db 100644
--- a/media/audio/audio_input_device.cc
+++ b/media/audio/audio_input_device.cc
@@ -312,6 +312,12 @@ void AudioInputDevice::AudioThreadCallback::MapSharedMemory() {
     audio_buses_.push_back(std::move(audio_bus));
     ptr += segment_length_;
   }
+
+  // Indicate that browser side capture initialization has succeeded and IPC
+  // channel initialized. This effectively completes the
+  // AudioCapturerSource::Start()' phase as far as the caller of that function
+  // is concerned.
+  capture_callback_->OnCaptureStarted();
 }
 
 void AudioInputDevice::AudioThreadCallback::Process(uint32_t pending_data) {