[CSS Shapes] Simplify Polygon implementation

Only floats can specify shape-outside. For polygon shape-outside values, the problem
of finding the left and right exclusion edges reduces to finding the X axis
limits of the polygon edges that overlap a particular layout line. This is a much
simpler version of the problem that PolygonShape was originally designed for, and so
the implementation can also be simplified. There's more about the new algorithm in
http://hansmuller-webkit.blogspot.com/2014/03/a-simpler-algorithm-for-css-shapes.html.

This is a merge of http://trac.webkit.org/changeset/166790 by Hans Muller <hmuller@adobe.com>.

BUG=360139

Review URL: https://codereview.chromium.org/226363003

git-svn-id: svn://svn.chromium.org/blink/trunk@170931 bbb929c8-8fbe-4397-9dbb-9b2b20218538