When using NavMeshComponents Surface script, building Height Mesh is not possible and the option is greyed out. But in Scene view the option to show HeightMesh checkbox is still active, when effectively it is useless. Suggestion was made to grey it out when there is no Legacy Height Mesh in the scene.

Steps to reproduce:
1. Open attached project.
2. Open Example scene 1_multiple_agent_sizes with Nav Mesh Surface script.
3. Bake NavMesh from NavMesh Surface scripts on Geometry gameObject.
3. Observe the lack of Height Mesh.
4. The checkbox in Scene view is still active.

Expected outcome: If there is no Height Mesh, the button does nothing, so it should be greyed out to not cause any confusion.
Actual outcome: You can check and uncheck the box with no effect.

Reproduced on 5.6.0f2 and 2017.1.0a4

Note: no repro on 5.5.x because it's a new feature.