Repaint background when using auto sizing for generated images

Generated backgrounds were not getting repainted when "background-size: auto"
was set and child elements changed height.

BUG=230398

Review URL: https://codereview.chromium.org/34033003

git-svn-id: svn://svn.chromium.org/blink/trunk@160198 bbb929c8-8fbe-4397-9dbb-9b2b20218538