Make GrAtlas::removePlot static

This CL just calves off a small chunk of (Add atlased layer purging - https://codereview.chromium.org/367073002/).

R=jvanverth@google.com
TBR=jvanverth@google.com

Author: robertphillips@google.com

Review URL: https://codereview.chromium.org/388103002
3 files changed