Fabric: Making `UIManager::getRelativeLayoutMetrics` safer
Fabric: Making `UIManager::getRelativeLayoutMetrics` safer

Summary:
In this method we have to maintain a retaining pointer to the node in order to access it as a raw pointer.
See also a comment in the codee.

Changelog: [Internal] Fabric-specific internal change.

Reviewed By: JoshuaGross

Differential Revision: D21464834

fbshipit-source-id: 69f6894009509e5feca291fab12c019208933816

facebook/react-native,janicduplessis/react-native,facebook/react-native,myntra/react-native,myntra/react-native,pandiaraj44/react-native,janicduplessis/react-native,pandiaraj44/react-native,pandiaraj44/react-native,facebook/react-native,hoangpham95/react-native,myntra/react-native,javache/react-native,facebook/react-native,arthuralee/react-native,hoangpham95/react-native,hoangpham95/react-native,hoangpham95/react-native,javache/react-native,javache/react-native,hoangpham95/react-native,pandiaraj44/react-native,arthuralee/react-native,arthuralee/react-native,janicduplessis/react-native,janicduplessis/react-native,pandiaraj44/react-native,facebook/react-native,arthuralee/react-native,hoangpham95/react-native,myntra/react-native,javache/react-native,hoangpham95/react-native,myntra/react-native,facebook/react-native,myntra/react-native,janicduplessis/react-native,janicduplessis/react-native,javache/react-native,myntra/react-native,javache/react-native,pandiaraj44/react-native,myntra/react-native,hoangpham95/react-native,facebook/react-native,facebook/react-native,pandiaraj44/react-native,arthuralee/react-native,janicduplessis/react-native,pandiaraj44/react-native,javache/react-native,javache/react-native,facebook/react-native,janicduplessis/react-native,javache/react-native,myntra/react-native