Revert of move erase into SkPixmap (patchset #1 id:1 of https://codereview.chromium.org/1161033005/)

Reason for revert:
broken builds

Original issue's description:
> move erase into SkPixmap
>
> harvested from https://codereview.chromium.org/1148793007
>
> BUG=skia:
> TBR=
>
> Committed: https://skia.googlesource.com/skia/+/0963f5dab079627c5523ce6a443af27a33e361f7

TBR=reed@google.com
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
BUG=skia:

Review URL: https://codereview.chromium.org/1157303008
4 files changed