Add tests where we split near an edge.

cc/quads/draw_polygon.cc

 // Copyright 2014 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "cc/quads/draw_polygon.h"
 
 #include <stddef.h>
 
 #include <vector>
 
@@ skipping 219 matching lines @@
 
   *is_coplanar = false;
   if (!neg_count) {
     *front = std::move(polygon);
     return;
   } else if (!pos_count) {
     *back = std::move(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;
   size_t pre_front_begin;
   size_t pre_back_begin;
 
   // Find the first vertex that is part of the front split polygon.
   front_begin = std::find_if(vertex_distance.begin(), vertex_distance.end(),
                              [](float val) { return val > 0.0; }) -
                 vertex_distance.begin();
@@ skipping 87 matching lines @@
     quads->push_back(
         gfx::QuadF(first,
                    gfx::PointF(points_[offset].x(), points_[offset].y()),
                    gfx::PointF(points_[op1].x(), points_[op1].y()),
                    gfx::PointF(points_[op2].x(), points_[op2].y())));
     offset = op2;
   }
 }
 
 }  // namespace cc