refactor EnumerateDevices to make it a persistent request.

content/common/media/media_stream_options.cc

Index: content/common/media/media_stream_options.cc
===================================================================
--- content/common/media/media_stream_options.cc	(revision 148913)
+++ content/common/media/media_stream_options.cc	(working copy)
@@ -24,4 +24,12 @@
       in_use(opened),
       session_id(kNoId) {}
 
+bool StreamDeviceInfo::operator==(const StreamDeviceInfo &other) const {
+  return stream_type == other.stream_type &&
+         name == other.name &&
+         device_id == other.device_id &&
+         in_use == other.in_use &&
+         session_id == other.session_id;
+}
+
 }  // namespace media_stream