[CSS Shapes] Add FloatPolygon unit tests

Added unit tests for empty polygons, simple polygons with colinear edges, simple rectilinear polygons, and simple polygons whose vertices are listed counter-clockwise.

BUG=329555

Review URL: https://codereview.chromium.org/101763005

git-svn-id: svn://svn.chromium.org/blink/trunk@164193 bbb929c8-8fbe-4397-9dbb-9b2b20218538