Revert "Rename IntRectExtent to IntBoxExtent"

Revert r169675 (commit aca3f9e60c129c49e4e48cc9097f80d7f697c417) as it
might have caused the Mac ASAN bot to fail.

R=vandebo@chromium.org
BUG=

Review URL: https://codereview.chromium.org/208073005

git-svn-id: svn://svn.chromium.org/blink/trunk@169760 bbb929c8-8fbe-4397-9dbb-9b2b20218538