Created Issue: [CORS] Request Id in CorsEngineTracer is null [964]

Following traces are what you get:

```
iisexpress.exe Information: 0 : [2013-04-02T23:33:47.6530772Z] Level=Info, Kind=Begin, Category='System.Web.Http.Cors', Id=00000000-0000-0000-0000-000000000000, Operation=CorsEngine.EvaluatePolicy

iisexpress.exe Information: 0 : [2013-04-02T23:33:47.6630854Z] Level=Info, Kind=End, Category='System.Web.Http.Cors', Id=00000000-0000-0000-0000-000000000000, Message='CorsResult returned: 'IsValid: True, AllowCredentials: False, PreflightMaxAge: 5, AllowOrigin: http://localhost:28793, AllowExposedHeaders: {}, AllowHeaders:
{dataserviceversion,accept,origin}, AllowMethods: {GET}, ErrorMessages: {}'', Operation=CorsEngine.EvaluatePolicy

```

Without request Id, it is difficult to correlate the trace with a given request.