Ensure text field hint line-height gets set correctly.

Call _fixupTextLayout when appending to the document and when hint is shown so the right line-height is applied.

_fixupTextLayout is called from didCreateLayer, but at that point the hint may be hidden and have an incorrect outerHeight().

Replaces #886. Rebased on top of master and pushed for convenient unit test run. Everything passes. Planned on merging it on in but decided to hold off pending a glance-over by @publickeating.