Remove IntSize::area, and rename IntSize::area_safe() to IntSize::area()

IntSize::area_safe was not named correctly for blink, and the old area
function should have been deprecated anyhow. This CL cleans this up.

Review URL: https://codereview.chromium.org/226523005

git-svn-id: svn://svn.chromium.org/blink/trunk@171363 bbb929c8-8fbe-4397-9dbb-9b2b20218538