Prefab Variant icon differs from prefab icon, however, when making an existing prefab to a prefab variant it's not getting changed.

Steps to Reproduce:
1. Ensure the Project window is set to one-column mode.
2. Create a GameObject in Hierarchy Window.
3. Drag it to Project Window to create a Prefab.
4. Open this newly created Prefab in Prefab Mode.
5. Ensure the Auto Save toggle is turned off in the Scene view Prefab bar.
6. Drag the root GameObject of Prefab into the Project window to create a new Prefab.
7. Select "Create Base" to create a new base for this prefab.
8. Project Window now has two prefabs.
9. Click the 'Save' button in the Scene view Prefab bar.
10. The Prefab that's open is now saved as a Prefab Variant and shows the correct icon in the Hierarchy titlebar.

Actual Result: Unable to differentiate between Prefab Variant and Base Prefab in the Project window.

Expected Result: Prefab Variant should have a different icon from Base Prefab in Project window.

Reproducible On: 2018.3.a2/e9113db2e9bf

Note:
- Appears on both OSX and Windows.
- Icon gets updated on creating a new Asset in Project Window, or simply by collapsing and expanding the foldout for the folder the Prefab with the wrong icon is in.