Accept 0px as a valid sizes value

This CL fixes a bug where a value of "0px" is not accepted as a valid
sizes value, and is replaced by the default "100vw".

BUG=418902

Review URL: https://codereview.chromium.org/632743005

git-svn-id: svn://svn.chromium.org/blink/trunk@183303 bbb929c8-8fbe-4397-9dbb-9b2b20218538