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This collapses most of the logic for painting composited
scrollbars into ScrollbarLayerDelegate. There was only one
path and lots of indirection before, most of which can now
go away.

This allows WebScrollbarImpl, WebScrollbarThemeClientImpl,
WebScrollbarThemeGeometryNative and WebScrollbarThemePainter
to be deleted. Bits of WebScrollbar remain, since there are
a handful of places that still use enums defined in its header.

Bug: 841751
Change-Id: Id37c75bc50e5138b44c5d6ba0add08f73ad71f0c
Reviewed-on: https://chromium-review.googlesource.com/1052919
Commit-Queue: Jeremy Roman <jbroman@chromium.org>
Reviewed-by: David Bokan <bokan@chromium.org>
Reviewed-by: Philip Rogers <pdr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#557591}