Move continuation outline painting logic into paintContinuationOutlines.

There is only one call site, in RenderBlock::paintObject. It has some
continuation outline painting logic at the call site; paintObject reads
more easily if this is moved inside.

BUG=302024

Review URL: https://codereview.chromium.org/410413009

git-svn-id: svn://svn.chromium.org/blink/trunk@178877 bbb929c8-8fbe-4397-9dbb-9b2b20218538