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This adds a ui::PaintRecorder that will be the gateway to the
PaintContext internals. The rules are:

1) Make a PaintRecorder when you want to access the canvas.
2) Can not nest PaintRecorders.

In the future PaintRecorder will also know how to deal with a
DisplayItemList and provider a Canvas that records to a DisplayItem
and each scoped PaintRecorder will produce DisplayItems to be cached.

For now, just restructuring code to go through PaintRecorder and not
nest recording.

R=sadrul, sky
BUG=466426

Review URL: https://codereview.chromium.org/1053143002

Cr-Commit-Position: refs/heads/master@{#323936}