Simplify the ShapeOutsideInfo and ShapeInfo interfaces

With the creation ShapeInfo::computeSegmentsForLine, it became possible
to simplify the ShapeInfo classes. ShapeOutsideInfo has no need to save
the segments, and now that segment creation is in its own method, it
doesn't have to save the segments anymore. This also simplifies the
public interface of ShapeOutsideInfo, as it only has one method to do
the computation.

Also, the methods in ShapeOutsideInfo have been renamed to reflect the
fact that ShapeOutsideInfo isn't really interested in segments.

Review URL: https://codereview.chromium.org/27073002

git-svn-id: svn://svn.chromium.org/blink/trunk@159501 bbb929c8-8fbe-4397-9dbb-9b2b20218538