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

[miu, 2017/01/11 20:51:11]

I don't think it's synchronous. The callback originates from
CastReceiverSessionDelegate::StartAudio(), which is running on the IO thread.
The code here is running on the UI thread.

Taking a step back: Note that ExternalMediaStreamAudioSource is already
constructed with a source. So, we don't have to worry about whether the source
exists. Therefore, it seems this class needs no changes since it should never
result in a device-not-found type of failure.

(Also, IMHO, you should define the
AudioCapturerSource::CaptureCallback::OnCaptureStarted() to be an
empty-by-default virtual method instead of a pure virtual method, to indicate
that handling this event is optional.)

[tommi (sloooow) - chröme, 2017/01/11 22:40:58]

OK, I traced through the code and may have missed something.  In order to
convince myself, I added the CHECK() below to see if I'd break any tests,
implemented OnCaptureStarted() below (note that when we get here, was_started_
will always be false).  So I guess this class isn't used in any test that runs
on the try or build bots?
Done.  Added documentation for things to consider as well and removed the
implementation from this file.

[miu, 2017/01/12 21:25:37]

IIRC, the original implementor added browser tests for Chrome's builtin Cast
streaming receiver (+ unit tests for the network+decode part of the impl). I
think they are built into the browser_tests binary.
SGTM.

+  // 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,