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 facebook#14111

Differential Revision: D5283942

Pulled By: sahrens

fbshipit-source-id: 8909532dfddd8628b7fb3380c198f0dfa88f240a

Libraries/Lists/VirtualizedList.js

@@ -1097,6 +1097,19 @@ class VirtualizedList extends React.PureComponent<OptionalProps, Props, State> {
     if (!frame || frame.index !== index) {
       if (getItemLayout) {
         frame = getItemLayout(data, index);
+        if (__DEV__) {
+          const frameType = PropTypes.shape({
+            length: PropTypes.number.isRequired,
+            offset: PropTypes.number.isRequired,
+            index: PropTypes.number.isRequired,
+          }).isRequired;
+          PropTypes.checkPropTypes(
+            {frame: frameType},
+            {frame},
+            'frame',
+            'VirtualizedList.getItemLayout'
+          );
+        }
       }
     }
     return frame;