Steps to reproduce:
1. Open user attached project (Wirefrime)
2. Open Scene.unity
3. Select shading mode to Shaded Wireframe
4. Observe that the wireframes do not appear on the terrain, even though it works perfectly fine on a Plane object

Expected result: shaded wireframe mode should display wireframes on the terrain
Actual result: shaded wireframe mode does not display wireframes on terrains

Reproduced in: 2017.2.0a4, 2017.1.0b10, 5.6.2f, 5.5.4p1, 5.5.0a6
Not reproduced in: 5.4.5p3
Regression since: 5.5.06

Fixed in: 2017.2.0b7