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 ed5e79d37e2ef5e49f4a07ff5b67eab141a425af..aa44d9d7552e0c4f40cf550465aa891fdf8edec9 100644
--- a/media/audio/audio_input_device.cc
+++ b/media/audio/audio_input_device.cc
@@ -292,6 +292,12 @@ 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) {