Summary:
Define getSizeWithHeaders in ImageLoader native module.
This diff is related to moving RCTImageViewManager's commands to a native module, ImageLoader.
See it's usage here: D17704091
Reviewed By: TheSavior
Differential Revision: D17693907
fbshipit-source-id: 3c2d7b19ac68ead831e780c4ee23e3ed0643be3a