GitLab

Don't use the canvas() from the PaintContext so that we can have the
PaintContext be backed by a DisplayItemList instead in the future.
Access to the Canvas should be done through a PaintRecorder.

R=piman@chromium.org, sadrul
BUG=466426

Review URL: https://codereview.chromium.org/1066183002

Cr-Commit-Position: refs/heads/master@{#324153}