Background image should clamp to minimum size(1, 1) after scale.

While one of the values of background-size is auto we have to use the
appropriate scale to maintain our aspect ratio. In this step we might
end up getting new image width or height as zero. In this case we should
clamp to minimum value as 1.

BUG=424048

Review URL: https://codereview.chromium.org/659913003

git-svn-id: svn://svn.chromium.org/blink/trunk@185238 bbb929c8-8fbe-4397-9dbb-9b2b20218538