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().
Removes the redundant fix up text layout call from didCreateLayer.
Adds unit tests to verify the bug fixed by #1126 / #886. BOOM!