Revert of Add constant color GrFP. (patchset #10 id:180001 of https://codereview.chromium.org/978713002/) Reason for revert: Revert while investigating assertions. Original issue's description: > Add constant color GrFP. > > Committed: https://skia.googlesource.com/skia/+/dfbbec436cbcacc3270d4b28357c8393e67d6494 TBR=egdaniel@google.com NOPRESUBMIT=true NOTREECHECKS=true NOTRY=true Review URL: https://codereview.chromium.org/1055023002