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

This reverts commit cca9b3ac908cf3e9a0c5774a5ffd118d8429e08a.

BUG=skia:
TBR=

Review URL: https://codereview.chromium.org/1153473012
4 files changed