[Effen] Try to catch more misuse of fn by asserting that UINode nodes are only built when we're updatin the tree.
[Effen] Try to catch more misuse of fn by asserting that UINode nodes are only built when we're updatin the tree.

R=abarth@chromium.org

Review URL: https://codereview.chromium.org/1148253002

chinmaygarde/mojo,jianglu/mojo,chinmaygarde/mojo,jianglu/mojo,jianglu/mojo,afandria/mojo,collinjackson/mojo,jianglu/mojo,collinjackson/mojo,chinmaygarde/mojo,afandria/mojo,afandria/mojo,collinjackson/mojo,jianglu/mojo,collinjackson/mojo,chinmaygarde/mojo,chinmaygarde/mojo,jianglu/mojo,jianglu/mojo,afandria/mojo,afandria/mojo,afandria/mojo,collinjackson/mojo,chinmaygarde/mojo,collinjackson/mojo,chinmaygarde/mojo,collinjackson/mojo,collinjackson/mojo,chinmaygarde/mojo,afandria/mojo,afandria/mojo,collinjackson/mojo,jianglu/mojo,chinmaygarde/mojo