Add unique ID to GrPlot

This is calved off of (Add atlased layer purging - https://codereview.chromium.org/367073002/) where it is used for plot lifetime management (i.e., tracking which plots are locked).

R=jvanverth@google.com

Author: robertphillips@google.com

Review URL: https://codereview.chromium.org/391153002
2 files changed