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..d355a1b06a296e5bbcb0f6084e36f282dfde0574 100644
--- a/media/base/audio_capturer_source.h
+++ b/media/base/audio_capturer_source.h
@@ -22,6 +22,14 @@ 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.
+    // This callback is provided for sources such as local audio sources that
+    // require asynchronous initialization so not all sources will support this
+    // notification.
+    virtual void OnCaptureStarted() {}
+
     // 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.