How to reproduce:
1. Create a new project
2. Create a cube
3. Drag and drop it to the project so it becomes a prefab
4. Select cube in the hierarchy and notice that Revert and Apply buttons are active
5. Open Timeline tab (Window > Timeline)
6. Create timeline for cube
7. Notice that Revert and Apply buttons are inactive

Actual result: Creating timeline for a prefab doesn't allow to apply or revert chages for the prefab
Expected result: Creating timeline for a prefab allows to apply or revert changes for the prefab. Just like drag and drop the object saves all the changes and creates a prefab

Reproducible: 2017.1.1f1, 2017.2.0b10, 2017.3.0a2, 2017.3.0a6