Avoid FF vs Chrome font size rounding differences

Choosing a point size that maps exactly to a pixel size avoids
differences between how FF and Chrome round font sizes. FF performs no
rounding, while Chrome rounds to the closest pixel.
Allow multiple accepted values in LayoutTests

Cope with cross-browser differences by allowing multiple possibly value
matches.
LayoutTests cleanup: use Object.keys array for key iteration
Add fixed line-height to ensure consistent cross-browser behaviour
Merge pull request #775 from peitschie/ff-font-size

Fix failing FF layout tests