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

content/renderer/media/external_media_stream_audio_source.cc

Index: content/renderer/media/external_media_stream_audio_source.cc
diff --git a/content/renderer/media/external_media_stream_audio_source.cc b/content/renderer/media/external_media_stream_audio_source.cc
index 7c2977c2907c46249730468bb2f63b81ffbe75a3..01b32ad6b3d46b57271791cb2cd645386f909cf9 100644
--- a/content/renderer/media/external_media_stream_audio_source.cc
+++ b/content/renderer/media/external_media_stream_audio_source.cc
@@ -40,7 +40,10 @@ bool ExternalMediaStreamAudioSource::EnsureSourceIsStarted() {
           << GetAudioParameters().AsHumanReadableString() << "}.";
   source_->Initialize(GetAudioParameters(), this, -1);
   source_->Start();
-  was_started_ = true;
+  // OnCaptureStarted() is expected to be called synchronously by this
+  // implementation. If this needs to be changed, the source needs to be started
+  // outside of EnsureSourceIsStarted since its design is synchronous.
+  CHECK(was_started_);
   return true;
 }
 
@@ -57,6 +60,10 @@ void ExternalMediaStreamAudioSource::EnsureSourceIsStopped() {
           << GetAudioParameters().AsHumanReadableString() << "}.";
 }
 
+void ExternalMediaStreamAudioSource::OnCaptureStarted() {
+  was_started_ = true;
+}
+
 void ExternalMediaStreamAudioSource::Capture(const media::AudioBus* audio_bus,
                                              int audio_delay_milliseconds,
                                              double volume,