To reproduce:

1. Open the project, attached by the tester (centerOfMass.zip)
2. Open the "scene" scene
3. Select ColliderAtRoot game object, the small red dot shows the center of mass
4. Set its scale to (1, 2, 1), observe the center of mass
5. Set its scale back to (1, 1, 1) and observer the center of mass

Expected: rigidbody.centerOfMass is recalculated properly

Reproduced in 2017.2.0a4, 2017.2.2p3, 2017.3.1f1, 2018.1.0f1, 2018.2.0a8
Did not reproduce in 2017.1.3p3, 2017.2.0a3
Regression introduced 2017.2.0a4