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This change moves the bulk of the Mac scrollbar drawing code into
NativeThemeMac. This is done in order to bring this more into
alignment with other platforms and to make it easier to transition things over to color pipeline.

It contains changes to do this for both the views UI and web platform ui.
CocoaScrollbarPainter is removed now that all of its previous responsibilities are in NativeThemeMac.

Bug: 1056950
Change-Id: Iedc224b582a7bcbdf920ed4b58855b284bfd0f10
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2195561Reviewed-by: David Bokan <bokan@chromium.org>
Reviewed-by: Alison Maher <almaher@microsoft.com>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Reviewed-by: Peter Kasting <pkasting@chromium.org>
Reviewed-by: Elly Fong-Jones <ellyjones@chromium.org>
Commit-Queue: Joan Barnett <johnsm@microsoft.com>
Cr-Commit-Position: refs/heads/master@{#770796}