Explicitly enforce values size in feColorMatrix.

R=senorblanco@chromium.org
BUG=468519

Review URL: https://codereview.chromium.org/1075413002

git-svn-id: svn://svn.chromium.org/blink/trunk@193571 bbb929c8-8fbe-4397-9dbb-9b2b20218538