Document PaintLayer self-painting concept
Document PaintLayer self-painting concept

This is a concept super important to painting/hit-testing and that
also underlines what's wrong with PaintLayer.

While touching the code, unified the style to match better what
is done in layout/.

Review URL: https://codereview.chromium.org/1390073007

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#353061}

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