Convert fast/repaint/background-scaling.html to a reftest

Originally the test expects full repaint of the viewport on page scale
changes. However, the repaint is unnecessary because we handle page
scale in the compositor. My later change will optimize this.

Convert it to a reftest to ensure the visual result is correct.

BUG=258219

Review URL: https://codereview.chromium.org/267333008

git-svn-id: svn://svn.chromium.org/blink/trunk@173658 bbb929c8-8fbe-4397-9dbb-9b2b20218538