Siqi Liu b5abb87a6f Fix the potential bug for network inspector when receiving xhr objects without an "_index" property
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
2016-07-28 14:13:46 -07:00
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