Enabling Metal support for Mac Editor unticks Auto Graphics API checkbox the first time Metal support is enabled.

Steps to reproduce:
1) Create a new project.
2) Go to PlayerSettings.
3) Tick 'Metal Editor Support* (Experimental)' checkbox.

Notice that 'Auto Graphics API for Mac' becomes unticked as well. If it is ticked again and 'Metal Editor Support* (Experimental)' is disabled and enabled again, the Graphics API checkbox remains ticked. This behaviour should be consistent - either always unticking Auto Graphics API, or leaving it unmodified at all.

Expected: 'Auto Graphics API for Mac' is not modified by Editor Metal Support checkbox.
Actual: 'Auto Graphics API for Mac' is unticked only the first time Editor Metal Support checkbox is ticked.

Reproduced on:
2017.2.0a1, 5.6.1p1, 5.6.0f3