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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.

For compositing effects (opacity) we use a CompositingRecorder to
provide a scoped opacity change.

R=pkasting
BUG=466426

Review URL: https://codereview.chromium.org/1062293003

Cr-Commit-Position: refs/heads/master@{#324173}