Revert of correct reporting of kAlphaUnchanged_Flag in ModeFilter (patchset #1 id...

Revert of correct reporting of kAlphaUnchanged_Flag in ModeFilter (patchset #1 id:1 of https://codereview.chromium.org/1033583002/)

Reason for revert:
reverting experiment. need to rebaseilne layouttest

svg/css/text-gradient-shadow.svg

Original issue's description:
> correct reporting of kAlphaUnchanged_Flag in ModeFilter
>
> BUG=skia:
> TBR=
>
> Committed: https://skia.googlesource.com/skia/+/afced66b508ac4cd3616e2f707db27b7d6f670e1

TBR=
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
BUG=skia:

Review URL: https://codereview.chromium.org/1026313002