Remove custom MAP_TAG from FileBuffer
Remove custom MAP_TAG from FileBuffer

Summary:
Custom tags made with `VM_MAKE_TAG` are meant to be used to tag
anonymous mappings, and should be passed in as the file descriptor.
We can't use them for file backed mappings.

Changelog: [Internal]

Reviewed By: kodafb

Differential Revision: D29038256

fbshipit-source-id: 11324a3e798e695709d7d340e84e5fe6f7ffaad3

facebook/react-native,janicduplessis/react-native,pandiaraj44/react-native,facebook/react-native,janicduplessis/react-native,pandiaraj44/react-native,pandiaraj44/react-native,facebook/react-native,javache/react-native,javache/react-native,pandiaraj44/react-native,facebook/react-native,facebook/react-native,javache/react-native,facebook/react-native,janicduplessis/react-native,javache/react-native,janicduplessis/react-native,javache/react-native,javache/react-native,pandiaraj44/react-native,facebook/react-native,pandiaraj44/react-native,facebook/react-native,javache/react-native,pandiaraj44/react-native,facebook/react-native,javache/react-native,pandiaraj44/react-native,janicduplessis/react-native,janicduplessis/react-native,janicduplessis/react-native,javache/react-native,janicduplessis/react-native