Pass a ui::PaintContext from ui::Layer to layer delegates.

This is a plumbing CL to create a ui::PaintContext that wraps a
gfx::Canvas up in ui::Layer. This is passed to every LayerDelegate,
and also through to aura::WindowDelegates and
views::NativeWidgetDelegates to eventually reach views::View as well.

This way ui::PaintContext is created in one place (for compositor
paints, there are other places painting to bitmaps we'll address
later). And in the future the ui::PaintContext can be backed by a
display list instead of a canvas.

R=sky
BUG=466426

Review URL: https://codereview.chromium.org/1057873004

Cr-Commit-Position: refs/heads/master@{#323855}

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