r.ScreenPercentage.Editor does not function

Setting r.ScreenPercentage.Editor does not affect the upsampling/downsampling of the viewport in the editor.

Regression:
YES - the console command correctly adjusted the screen precentage in 4.11.2

Broken version - Binary 4.12.0 - CL 2992821
Working version - Binary 4.11.2 - CL 2946394

1. Open UE4 Editor (any project)
2. Open console command window and enter "r.ScreenPercentage.Editor 1"
3. Add post process volume to the level
4. In details panel
   • Find and check the box for Unbound
   • Find and check the box for Screen Percentage
   • Adjust the slider up and down

Result:
No change to the viewport

Expected:
Viewport blurs/sharpens as the slider is moved to the left/right (respectively)

Repro Rate:
3/3