refactor EnumerateDevices to make it a persistent request.

content/browser/renderer_host/media/media_stream_dispatcher_host.cc

Index: content/browser/renderer_host/media/media_stream_dispatcher_host.cc
===================================================================
--- content/browser/renderer_host/media/media_stream_dispatcher_host.cc	(revision 148913)
+++ content/browser/renderer_host/media/media_stream_dispatcher_host.cc	(working copy)
@@ -100,10 +100,9 @@
   StreamMap::iterator it = streams_.find(label);
   DCHECK(it != streams_.end());
   StreamRequest request = it->second;
-  streams_.erase(it);
 
   Send(new MediaStreamMsg_DevicesEnumerated(
-      request.render_view_id, request.page_request_id, devices));
+      request.render_view_id, request.page_request_id, label, devices));
 }
 
 void MediaStreamDispatcherHost::DevicesEnumerationFailed(
@@ -226,7 +225,6 @@
            << ", {label = " << label <<  "})";
 
   StreamMap::iterator it = streams_.find(label);
-  DCHECK(it != streams_.end());
   GetManager()->StopGeneratedStream(label);
   streams_.erase(it);
 }