The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/7205053587e2253f8631ebd3ed75296c0c58f866

commit 7205053587e2253f8631ebd3ed75296c0c58f866
Author: Philip Rogers <pdr@chromium.org>
Date: Fri Oct 13 23:53:00 2017

Use snapped size for scrollbar thumb geometry

Scrollbar thumb size is set through Scrollbar::SetProportion.
PaintLayerScrollableArea set this with a truncated overflow
rect size, whereas pixel snapped sizes are used elsewhere (e.g.,
MaximumScrollOffsetInt). This patch switches PLSA's scrollbar
thumb geometry to use ContentsSize which snaps instead of
truncates.

This patch exposed a bug in unscrollable-subpixel-overflow-mouse-drag
where the test actually is scrollable but was returning no
scroll offset due to truncation.

Bug:  773377 
Cq-Include-Trybots: master.tryserver.chromium.linux:linux_layout_tests_slimming_paint_v2
Change-Id: I6ee276865c42941dba0d3dacdb63b94bef87b80c
Reviewed-on: https://chromium-review.googlesource.com/714457
Reviewed-by: Steve Kobes <skobes@chromium.org>
Commit-Queue: Philip Rogers <pdr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#508875}
[modify] https://crrev.com/7205053587e2253f8631ebd3ed75296c0c58f866/third_party/WebKit/LayoutTests/FlagExpectations/root-layer-scrolls
[delete] https://crrev.com/a2746bc31443102369bea08eb3cbb06de91a053b/third_party/WebKit/LayoutTests/compositing/overflow/nested-render-surfaces-with-rotation-expected.png
[modify] https://crrev.com/7205053587e2253f8631ebd3ed75296c0c58f866/third_party/WebKit/LayoutTests/fast/forms/select/listbox-overlay-scrollbar-expected.html
[add] https://crrev.com/7205053587e2253f8631ebd3ed75296c0c58f866/third_party/WebKit/LayoutTests/fast/scrolling/subpixel-overflow-mouse-drag-expected.txt
[rename] https://crrev.com/7205053587e2253f8631ebd3ed75296c0c58f866/third_party/WebKit/LayoutTests/fast/scrolling/subpixel-overflow-mouse-drag.html
[delete] https://crrev.com/a2746bc31443102369bea08eb3cbb06de91a053b/third_party/WebKit/LayoutTests/fast/scrolling/unscrollable-subpixel-overflow-mouse-drag-expected.txt
[modify] https://crrev.com/7205053587e2253f8631ebd3ed75296c0c58f866/third_party/WebKit/LayoutTests/platform/mac-mac10.10/fast/forms/select/listbox-appearance-basic-expected.png
[modify] https://crrev.com/7205053587e2253f8631ebd3ed75296c0c58f866/third_party/WebKit/LayoutTests/platform/mac-mac10.10/fast/forms/select/select-item-background-clip-expected.png
[modify] https://crrev.com/7205053587e2253f8631ebd3ed75296c0c58f866/third_party/WebKit/LayoutTests/platform/mac-mac10.10/paint/invalidation/textarea-caret-expected.png
[modify] https://crrev.com/7205053587e2253f8631ebd3ed75296c0c58f866/third_party/WebKit/LayoutTests/platform/mac-mac10.11/fast/forms/select/listbox-appearance-basic-expected.png
[modify] https://crrev.com/7205053587e2253f8631ebd3ed75296c0c58f866/third_party/WebKit/LayoutTests/platform/mac-mac10.11/fast/forms/select/select-item-background-clip-expected.png
[modify] https://crrev.com/7205053587e2253f8631ebd3ed75296c0c58f866/third_party/WebKit/LayoutTests/platform/mac-retina/fast/forms/select/listbox-appearance-basic-expected.png
[modify] https://crrev.com/7205053587e2253f8631ebd3ed75296c0c58f866/third_party/WebKit/LayoutTests/platform/mac-retina/fast/forms/select/select-item-background-clip-expected.png
[add] https://crrev.com/7205053587e2253f8631ebd3ed75296c0c58f866/third_party/WebKit/LayoutTests/platform/mac/compositing/overflow/nested-render-surfaces-with-rotation-expected.png
[modify] https://crrev.com/7205053587e2253f8631ebd3ed75296c0c58f866/third_party/WebKit/LayoutTests/platform/mac/fast/forms/select/listbox-appearance-basic-expected.png
[modify] https://crrev.com/7205053587e2253f8631ebd3ed75296c0c58f866/third_party/WebKit/LayoutTests/platform/mac/fast/forms/select/select-item-background-clip-expected.png
[modify] https://crrev.com/7205053587e2253f8631ebd3ed75296c0c58f866/third_party/WebKit/LayoutTests/platform/mac/fast/overflow/childFocusRingClip-expected.png
[modify] https://crrev.com/7205053587e2253f8631ebd3ed75296c0c58f866/third_party/WebKit/LayoutTests/platform/mac/fast/overflow/overflow-text-hit-testing-expected.png
[modify] https://crrev.com/7205053587e2253f8631ebd3ed75296c0c58f866/third_party/WebKit/LayoutTests/platform/mac/paint/invalidation/overflow-outline-repaint-expected.png
[add] https://crrev.com/7205053587e2253f8631ebd3ed75296c0c58f866/third_party/WebKit/LayoutTests/platform/mac/virtual/prefer_compositing_to_lcd_text/compositing/overflow/nested-render-surfaces-with-rotation-expected.png
[modify] https://crrev.com/7205053587e2253f8631ebd3ed75296c0c58f866/third_party/WebKit/LayoutTests/platform/win/compositing/overflow/nested-render-surfaces-with-rotation-expected.png
[modify] https://crrev.com/7205053587e2253f8631ebd3ed75296c0c58f866/third_party/WebKit/LayoutTests/platform/win/fast/overflow/007-expected.png
[modify] https://crrev.com/7205053587e2253f8631ebd3ed75296c0c58f866/third_party/WebKit/LayoutTests/platform/win/virtual/prefer_compositing_to_lcd_text/compositing/overflow/nested-render-surfaces-with-rotation-expected.png
[delete] https://crrev.com/a2746bc31443102369bea08eb3cbb06de91a053b/third_party/WebKit/LayoutTests/virtual/prefer_compositing_to_lcd_text/compositing/overflow/nested-render-surfaces-with-rotation-expected.png
[modify] https://crrev.com/7205053587e2253f8631ebd3ed75296c0c58f866/third_party/WebKit/Source/core/paint/PaintLayerScrollableArea.cpp
[modify] https://crrev.com/7205053587e2253f8631ebd3ed75296c0c58f866/third_party/WebKit/Source/core/paint/PaintLayerScrollableArea.h