Rename IntRectExtent to IntBoxExtent

Rename the IntRectExtent class to IntBoxExtent to match the naming of
the LayoutUnit and floating point versions of the same concept.

R=leviw@chromium.org

Review URL: https://codereview.chromium.org/206373006

git-svn-id: svn://svn.chromium.org/blink/trunk@169675 bbb929c8-8fbe-4397-9dbb-9b2b20218538