TextInputWidget: Don't mix inner and outer heights in calculations

Caused IE 11 to display a vertical scrollbar when the textarea was two
rows high (the default height).

Also tweaks.

Bug: 71310
Change-Id: I8ba9af7f63b14f75bea805384f13b6b2721a8f10
Merge "TextInputWidget: Don't mix inner and outer heights in calculations"