Fix the rendering error of SkDraw::drawVertices in gpu path for solid color

include/core/SkCanvas.h

Index: include/core/SkCanvas.h
diff --git a/include/core/SkCanvas.h b/include/core/SkCanvas.h
index 70cfe287b85bb9226803dbdaf045db185eeb96a8..0e8265dba47213892ef08948bc5c8f3bab311293 100644
--- a/include/core/SkCanvas.h
+++ b/include/core/SkCanvas.h
@@ -1002,6 +1002,11 @@ public:
     };
 
     /** Draw the array of vertices, interpreted as triangles (based on mode).
+
+        If both textures and vertex-colors are NULL, we strokes hairlines with
+        the paint's color. This behavior is a useful debugging mode to visualize
+        the mesh.
+
         @param vmode How to interpret the array of vertices
         @param vertexCount The number of points in the vertices array (and
                     corresponding texs and colors arrays if non-null)