Add tests where we split near an edge.

cc/quads/draw_polygon.cc

Index: cc/quads/draw_polygon.cc
diff --git a/cc/quads/draw_polygon.cc b/cc/quads/draw_polygon.cc
index 86cb87319426d18f5ee1a1ddc3c6839d994ab289..59ecbef0edbacff11765138e2edf126695cfdfd6 100644
--- a/cc/quads/draw_polygon.cc
+++ b/cc/quads/draw_polygon.cc
@@ -237,10 +237,6 @@ void DrawPolygon::SplitPolygon(std::unique_ptr<DrawPolygon> polygon,
     return;
   }
 
-  // There should be at most two points that are considered to be on the thick
-  // plane. If this is not the case, then the polygon is not convex.
-  DCHECK_LE(num_points - pos_count - neg_count, 2u);
-
   // Handle splitting case.
   size_t front_begin;
   size_t back_begin;