We'll use this function to position the caret when the user taps a text
input control.
Very little of the code in this patch is actually new. Most of it is
restoring code that we previously removed from the engine. I've made
some small changes to the restored code to handle the lack of a DOM. The
only major change is to RenderObject::createPositionWithAffinity, which
now just returns the values it captures instead of trying to compute a
DOM position.
TextAffinity and TextPosition are lifted from package:flutter. Once this
patch rolls into package:flutter, I'll remove the declarations there.
