Fixes https://github.com/flutter/flutter/issues/14555

Now lib/ui depends on sky/engine rather than the reverse.

This removes most (all?) of our dependency on base::RefCountedThreadSafe.

* Adopt //lib/tonic/logging

* Adopt //lib/tonic/scopes

* Adopt //lib/tonic/parsers

Also, switch over to //lib/tonic's typed data classes.

Hopefully this will build this time.

Now that tonic doesn't depend on anything in //sky/engine anymore, we
can move the code to a location where its dependencies are clearer.

TextBox is a rect that understands TextDirection.