[CSS Shapes] Image shape-outside with vertical gaps is handled incorrectly

RasterShapeIntervals::getExcludedIntervals() was returning an empty list when the line overlapped any vertical gap in the image. This short-circuit had been mistakenly copied from getIncludedIntervals(), where it makes sense.

BUG=315918

Review URL: https://codereview.chromium.org/63623002

git-svn-id: svn://svn.chromium.org/blink/trunk@161542 bbb929c8-8fbe-4397-9dbb-9b2b20218538