Revert of Use presence of a content key as non-scratch indicator (patchset #5 id...

Revert of Use presence of a content key as non-scratch indicator (patchset #5 id:80001 of https://codereview.chromium.org/639873002/)

Reason for revert:
breaking nanobench on ubuntu

Original issue's description:
> Use presence of a content key as non-scratch indicator
>
> BUG=skia:2889
>
> Committed: https://skia.googlesource.com/skia/+/9eefe0851eeaa8ded05b4774ebcb38ed201d5dbf

TBR=robertphillips@google.com
NOTREECHECKS=true
NOTRY=true
BUG=skia:2889

Review URL: https://codereview.chromium.org/642493003