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This cleans up some of the damage rects code by converting the SkRegion
to a cc::Region, allowing use of gfx::Rects. It moves the recursion
over the Layer tree out to Compositor instead of on Layer.

And we keep the damaged_region_ valid until the layer is painted. This
will allow us to pass that damaged_region_ to the painting code with
impl-side slimming paint, because with impl-side painting, the paint
clip rect can be larger than the invalidations (as large as the whole
layer).

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

Committed: https://crrev.com/a5e585868e16ce53c990f764d4943592f11749aa
Cr-Commit-Position: refs/heads/master@{#326547}

Review URL: https://codereview.chromium.org/1080633009

Cr-Commit-Position: refs/heads/master@{#326912}