Implement GetUserMedia device ID constraint processing in the renderer.

content/common/media/media_stream_options.h

Index: content/common/media/media_stream_options.h
diff --git a/content/common/media/media_stream_options.h b/content/common/media/media_stream_options.h
index 2eb5628342534ddd987023bfc011634a1824388d..55f568736b9480492aacddf0d31ac9bfd58ebeac 100644
--- a/content/common/media/media_stream_options.h
+++ b/content/common/media/media_stream_options.h
@@ -33,17 +33,13 @@ struct CONTENT_EXPORT TrackControls {
   // Consider replacing with MediaStreamType enum variables.
   std::string stream_source;  // audio.kMediaStreamSource
 
-  // Device ID requests.
-  // The first set represents required devices - either grab one or fail.
-  // The second set represents optional devices - if we can't get one of
-  // these, we will grab the default device (if possible).
-  // The constraint names are "sourceId" and "chromeMediaSourceId".
-  std::vector<std::string> device_ids;
-  std::vector<std::string> alternate_device_ids;
+  // An empty string represents the default device.
+  // A nonempty string represents a specific device.
+  std::string device_id;
 };
 
 // StreamControls describes what is sent to the browser process
-// to the renderer process in order to control the opening of a device
+// from the renderer process in order to control the opening of a device
 // pair. This may result in opening one audio and/or one video device.
 // This has to be a struct with public members in order to allow it to
 // be sent in the IPC of media_stream_messages.h