Fabric: Calling JSVM GC on memory pressure event on iOS (second attempt)
Fabric: Calling JSVM GC on memory pressure event on iOS (second attempt)

Summary:
This change is especially important for Fabric when a lot of objects (mostly ShadowNodes) have shared ownership. Without this change, JSVM could not know that bunch of natively allocated objects should be deallocated.
Changelog: [Internal] Fabric-specific internal change.

Reviewed By: mdvacca

Differential Revision: D21798527

fbshipit-source-id: f2051721b074b99660cdfd6c5b75679b9792403e

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