Janic Duplessis a2b0ee0fc3 FlatList - Add dev validation of the object returned by getItemLayout
Summary:
Returning an object that doesn't have all the required properties from `getItemLayout` doesn't cause a flow error (maybe because we are using `createAnimatedComponent`) and caused ALL items to be rendered which caused perf issues that were hard to debug (typo lenght -> length -_-). This adds a simple warning in DEV mode using checkPropTypes.

**Test plan**
Tested in RNTester by passing a bad `getItemLayout` function.
![image](https://cloud.githubusercontent.com/assets/2677334/26329030/5b32ba90-3f13-11e7-9190-08f05a5c0682.png)
Closes https://github.com/facebook/react-native/pull/14111

Differential Revision: D5283942

Pulled By: sahrens

fbshipit-source-id: 8909532dfddd8628b7fb3380c198f0dfa88f240a
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