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

media/base/audio_capturer_source.h

Index: media/base/audio_capturer_source.h
diff --git a/media/base/audio_capturer_source.h b/media/base/audio_capturer_source.h
index d4f9cabb6c535b801cc52851fe47d12d9bac1584..9b6f8e8fa595b3bceec1fd9baa953824251a65cc 100644
--- a/media/base/audio_capturer_source.h
+++ b/media/base/audio_capturer_source.h
@@ -22,6 +22,11 @@ class AudioCapturerSource
  public:
   class CaptureCallback {
    public:
+    // Signals that audio recording has been started.  Called asynchronously
+    // after Start() has completed. If Start() encounters problems before this
+    // callback can be made, OnCaptureError will be called instead.
+    virtual void OnCaptureStarted() = 0;

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

Since not all implementations need this, consider making it an empty method
instead of pure virtual.

[tommi (sloooow) - chröme, 2017/01/11 22:40:59]

Done.

+
     // Callback to deliver the captured data from the OS.
     // TODO(chcunningham): Update delay argument to use frames instead of
     // milliseconds to prevent loss of precision. See http://crbug.com/587291.