Currently this is a pure refactor and doesn't change any functionality.
In a follow-up we can us this bit to mask out DS RTs when Manhattan and
other apps render with feedback loops.

Bug: angleproject:4517
Change-Id: I80ccd022d90a781506791110d11be195db8cd3e9
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2112936Reviewed-by: Cody Northrop <cnorthrop@google.com>