Use ComputeNormals in vtkContourFilter.

The existing implementation ignored this setting. Changed the
default ComputeNormals setting to preserve old behavior unless
explicitly set by the user.

Change-Id: Ie599c4853881fa201509de5d835202b43967b474
Merge topic 'contour-normals' into master

3b6cb421 Use ComputeNormals in vtkContourFilter.