[CSS Regions][CSS Shapes] Update updateShapeAndSegmentsForCurrentLineInFlowThread to deal better with multiple regions

I simplified the determination of the next region part of updateShapeAndSegmentsForCurrentLineInFlowThread's implementation
in order to make the code more straightforward. I also tried to avoid using regionAtBlockOffset(...) function where it's possible,
since it's not always that reliable, what I'll will report in a separate bug.

Merged WebKit revision: http://trac.webkit.org/changeset/158016

Review URL: https://codereview.chromium.org/50423005

git-svn-id: svn://svn.chromium.org/blink/trunk@160881 bbb929c8-8fbe-4397-9dbb-9b2b20218538