[CSS Regions][CSS Shapes] Layout error when the shape has negative top coordinate and it's applied on the second region

We have a layout error when there is a shape applied on the second region, and it has a negative 'top' coordinate.
Since shapeInsideInfo::shapeLogicalTop() can return negative numbers, we need to check for it, when we're positioning
the first line in the region.

Merged WebKit revision: http://trac.webkit.org/changeset/158219

Review URL: https://codereview.chromium.org/53103003

git-svn-id: svn://svn.chromium.org/blink/trunk@160984 bbb929c8-8fbe-4397-9dbb-9b2b20218538