Cleanup: Use WTF_ARRAY_LENGTH instead of homespun sizeof; update comment.

R=dglazkov@chromium.org

Review URL: https://codereview.chromium.org/18977004

git-svn-id: svn://svn.chromium.org/blink/trunk@154156 bbb929c8-8fbe-4397-9dbb-9b2b20218538