Summary: This adds a callback for <Text> to get metrics about the rendered text. It's divided by line but that could be changed to "fragments" (which makes more sense for multi-lingual). Right now by line is convenient as you frequently want to know where the first and last line end (though we could make this work with fragments I suppose).
Reviewed By: shergin
Differential Revision: D9440914
fbshipit-source-id: bb011bb7a52438380d3f604ffe7019b98c18d978
64a52532fe