[CSS Shapes] Add FloatPolygon unit tests

Test the basic functionality of the FloatPolygon class.  

Found and corrected a bug in the FloatPolygon::contains() method, when WindRule = RULE_NONZERO. Points at the vertical limits of the polygon's bounding box but not within the polygon, didn't contribute to the winding number. Now they do.

BUG=329052

Review URL: https://codereview.chromium.org/117703002

git-svn-id: svn://svn.chromium.org/blink/trunk@164047 bbb929c8-8fbe-4397-9dbb-9b2b20218538