Services Tab is opened by default after restarting an engine even if another Tab was left opened before closing Unity.

Steps to reproduce:
1. Create a New project
2. Open Services Tab (make sure Services Tab is docked)
3. Open any other Tab (for example, Inspector Tab)
4. Quit unity and open it again

Expected behavior: the Tab that was left opened before closing Unity shows up.
Actual result: Services Tab shows up.

Reproduced with: 5.6.0a3, 5.5.0b11, 5.4.2f1,
Not reproduced with: 5.3.7p1, 5.2.5f1

Note: reproduced on Mac and Windows.