Deduplicate PaintScrollbar

CompositedLayerMapping.cpp and PaintLayerCompositor.cpp has almost
identical (differs by a null-check) PaintScrollbar helpers.
Move a cleaned up version to ScrollableAreaPainter (being the least bad
common location.)
This fixes Jumbo compilation.

Bug: 713137
Cq-Include-Trybots: master.tryserver.chromium.linux:linux_layout_tests_slimming_paint_v2
Change-Id: Icc36e57ed364b18716fd56b0363431f352ef0d72
Reviewed-on: https://chromium-review.googlesource.com/741595
Commit-Queue: Xianzhu Wang <wangxianzhu@chromium.org>
Reviewed-by: Xianzhu Wang <wangxianzhu@chromium.org>
Reviewed-by: Philip Rogers <pdr@chromium.org>
Reviewed-by: Daniel Bratell <bratell@opera.com>
Cr-Commit-Position: refs/heads/master@{#512224}
4 files changed