This exposes the last hooks needed to implement flexbox layout. For now,
I didn't worry too much about the exact API we're exposing since this will
all change with the upcoming redesign (e.g. https://codereview.chromium.org/1093633002).
minContentWidth == the width if the element were to wrap at every wrapping point (not including border/padding)
maxContentWidth == the width if the element were to only wrap at hard wrapping points (e.g. \n inside a whitespace: pre).
R=eseidel@chromium.org
Review URL: https://codereview.chromium.org/1101793003
fc1dff4288