Summary:
Changelog: [Internal]
The main use-case here is to get the rootTag off RCTImageView, for image loading instrumentation. The fact is, each RCTView subclass already has `reactTag` attached today. We already have the `rootTag` when the view is created by the UIManager, so why not just attach it like reactTag? If we don't, looking up the rootTag from the native component is non-trivial and extremely inefficient (have to jump to shadow queue, back to main queue, etc).
Reviewed By: sammy-SC
Differential Revision: D18497002
fbshipit-source-id: 8409e3a1c95e09accedd959592cbf178fab0b2c3