Steps to reproduce:
1. Open QA attached project "NavMeshSamplePosition.zip"
2. Open Asset scene and locate the Cube object
3. Enter play mode
4. Open the Navigation tab to see the NavMesh preview
5. Move sideways along the X axis between the two NavMeshes (As per repro video)
6. SamplePosition returns the top level NavMesh only when it's directly above, even if it's closer than the one at the bottom

Expected: Sample Position should return the closest point from the source to the NavMesh regardless of positioning
Actual: Sample Position produces unreliable results when there are multiple levels of NavMesh

Reproduced on: 2017.1.0b9, 17.1.0b10, 17.1.0f3, 17.1.0p3, 17.2.0b6, 17.3.0a3
Regression introduced in: 2017.1.0b8
Not reproduced on: 5.6.3f1, 2017.1.0b1, 17.1.0b5, 17.1.0b7