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This removes the class, replacing it with uses of the cc scrollbar layer
types: cc::PaintedScrollbarLayer, cc::PaintedOverlayScrollbarLayer, and
cc::SolidColorScrollbarLayer directly. The scrollbar layer map needs to
maintain ownership of these along with a wrapping WebLayer (for now),
and a raw pointer to a base cc::ScrollbarLayerInterface* for changing
the element id that the scrollbar controls the view of. So we add a
ScrollingCoordinator::ScrollbarLayerGroup that holds these three things
together.

The cc_blink::ScrollbarImpl, which is a subclass of cc::Scrollbar, moves
from cc/blink/ to platform/scroll/ beside the ScrollingCoordinator, and
is renamed to ScrollbarLayerDelegate. It still makes use of Web* types
like WebScrollbarThemePainter and these can be removed to use the
platform/scroll/ types directly instead.

R=bokan@chromium.org, jbroman@chromium.org

Bug: 838693, 738465
Cq-Include-Trybots: luci.chromium.try:android_optional_gpu_tests_rel;master.tryserver.blink:linux_trusty_blink_rel;master.tryserver.chromium.linux:linux_layout_tests_slimming_paint_v2
Change-Id: I021fc69a2c8fd90a21bf406cd08d968e0335506a
Reviewed-on: https://chromium-review.googlesource.com/1048247Reviewed-by: Philip Rogers <pdr@chromium.org>
Reviewed-by: Jeremy Roman <jbroman@chromium.org>
Reviewed-by: David Bokan <bokan@chromium.org>
Commit-Queue: danakj <danakj@chromium.org>
Cr-Commit-Position: refs/heads/master@{#557203}