Tickmarks for Aura Overlay Scrollbars

Currently Aura Overlay Scrollbar paints in PaintedOverlayScrollbarLayer which
only paint the thumb part. In this patch, we paint the tickmarks when
we have tickmarks in scrollbar when PaintedOverlayScrollbarLayer update.

Bug: 759152
Cq-Include-Trybots: master.tryserver.blink:linux_trusty_blink_rel;master.tryserver.chromium.android:android_optional_gpu_tests_rel
Change-Id: I9e351b475cd9afa14cdae9264a2b78ab36a18258
Reviewed-on: https://chromium-review.googlesource.com/641611
Commit-Queue: Jianpeng Chao <chaopeng@chromium.org>
Reviewed-by: Rick Byers <rbyers@chromium.org>
Reviewed-by: weiliangc <weiliangc@chromium.org>
Reviewed-by: David Bokan <bokan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#540284}

• cc/BUILD.gn[diff]
• cc/blink/scrollbar_impl.cc[diff]
• cc/blink/scrollbar_impl.h[diff]
• cc/input/scrollbar.h[diff]
• cc/layers/painted_overlay_scrollbar_layer.cc[diff]
• cc/layers/painted_overlay_scrollbar_layer.h[diff]
• cc/layers/painted_overlay_scrollbar_layer_impl.cc[diff]
• cc/layers/painted_overlay_scrollbar_layer_impl.h[diff]
• cc/layers/painted_overlay_scrollbar_layer_unittest.cc[Added - diff]
• cc/test/fake_scrollbar.cc[diff]
• cc/test/fake_scrollbar.h[diff]
• cc/trees/layer_tree_host_pixeltest_scrollbars.cc[diff]
• third_party/WebKit/Source/platform/exported/WebScrollbarImpl.cpp[diff]
• third_party/WebKit/Source/platform/exported/WebScrollbarImpl.h[diff]
• third_party/WebKit/public/platform/WebScrollbar.h[diff]