Summary: It sometimes happens that there are already some existing `XMLHttpRequest` objects before we turn on the network inspector. So it is a must to check whether a `XMLHttpRequest` object has a property `_index` to determine if it should be tracked.
Reviewed By: davidaurelio
Differential Revision: D3635184
fbshipit-source-id: a5552d7244c994b0fe782ac35baae39ec7488494