Summary:
This diff deprecates `-[RCTSurface rootViewTag]` in favour of `-[RCTSurface rootTag]` which returns
same value but represented as `ReactTag` (`long`) instead of `NSNumber *`.
Reviewed By: mdvacca
Differential Revision: D7857044
fbshipit-source-id: f5427c30d3de2d3563e830f2caac70b7dc9631f9