Calculate the correct available height for content

The available height for content is the interior of the content box, not
the border box.

http://www.w3.org/TR/CSS2/visudet.html#propdef-height
http://www.w3.org/TR/CSS2/visudet.html#inline-replaced-height

BUG=236043

Review URL: https://codereview.chromium.org/407753002

git-svn-id: svn://svn.chromium.org/blink/trunk@178693 bbb929c8-8fbe-4397-9dbb-9b2b20218538