always compute a cubic normal

gm/bug615686.cpp

Index: gm/bug615686.cpp
diff --git a/gm/bug615686.cpp b/gm/bug615686.cpp
new file mode 100644
index 0000000000000000000000000000000000000000..1bc444202a73fb8697e68494fd5400183e09a007
--- /dev/null
+++ b/gm/bug615686.cpp
@@ -0,0 +1,20 @@
+/*
+ * Copyright 2016 Google Inc.
+ *
+ * Use of this source code is governed by a BSD-style license that can be
+ * found in the LICENSE file.
+ */
+
+#include "gm.h"
+#include "SkPath.h"
+
+DEF_SIMPLE_GM(bug615686, canvas, 250, 250) {
+    SkPaint p;
+    p.setAntiAlias(true);
+    p.setStyle(SkPaint::kStroke_Style);
+    p.setStrokeWidth(20);
+    SkPath path;
+    path.moveTo(0, 0);
+    path.cubicTo(200, 200, 0, 200, 200, 0);
+    canvas->drawPath(path, p);
+}