This regressed sometime in the past month (r473100 - r486026).  I'm bisecting now.

I'd be inclined to ignore / rebaseline these if it was just one or two tests, but when it's literally every test with any amount of text the baselines will cause a lot of pain.

Bisect implicates http://crrev.com/481209 ("Force rootScroller to create composited scroll layers").  Still investigating how - @bokan do you have any idea?

Found the cause - pages without overflow are having their backgrounds painted into the main GraphicsLayer instead of the scrolling contents layer.

Fix in progress.

(The manifestation of this bug in Chrome is that we lose subpixel antialiasing.  It's a blessing in disguise that content_shell caught this despite not doing subpixel AA.)

Yes, I missed that consequence in the patch, sorry about that and thanks for picking it up! The fix then is to use the paint background into scrolling content layer path, yes?

Curious, content_shell doesn't do subpixel AA? Any idea where the diffs come from in that case? 

Yep, the fix is to paint the background into the scrolling contents layer.

By default I get subpixel AA from chrome but only grayscale AA from content_shell.  I haven't figured out if there is some flag to get content_shell to do it (--enable-lcd-text doesn't work).

In theory grayscale AA on a transparent background with a white layer behind it should be identical to grayscale AA on a white background, but in practice some pixels are just slightly off.  I'm guessing it's floating-point rounding errors somewhere in the compositing pipeline.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/9e0775c12ad128945061bcd5bf1265ab1e90b4d3

commit 9e0775c12ad128945061bcd5bf1265ab1e90b4d3
Author: Steve Kobes <skobes@chromium.org>
Date: Tue Jul 18 19:36:42 2017

Let backgrounds paint in scrolling contents layer without overflow.

Since r481209, the document creates composited scrolling layers with RLS even if
it doesn't overflow.  But PaintLayer::GetBackgroundPaintLocation didn't get the
memo.

Bug:  741858 
Change-Id: I1fe09532eecc2bbd1b71c42e171bf86f4a893382
Reviewed-on: https://chromium-review.googlesource.com/570940
Reviewed-by: David Bokan <bokan@chromium.org>
Commit-Queue: Steve Kobes <skobes@chromium.org>
Cr-Commit-Position: refs/heads/master@{#487557}
[delete] https://crrev.com/b650c38e592cc27ed3572a8ef6d4290b9457e98f/third_party/WebKit/LayoutTests/platform/linux/virtual/rootlayerscrolls/scrollbars/basic-scrollbar-expected.png
[delete] https://crrev.com/b650c38e592cc27ed3572a8ef6d4290b9457e98f/third_party/WebKit/LayoutTests/platform/linux/virtual/rootlayerscrolls/scrollbars/border-box-rect-clips-scrollbars-expected.png
[delete] https://crrev.com/b650c38e592cc27ed3572a8ef6d4290b9457e98f/third_party/WebKit/LayoutTests/platform/linux/virtual/rootlayerscrolls/scrollbars/custom-scrollbar-appearance-property-expected.png
[delete] https://crrev.com/b650c38e592cc27ed3572a8ef6d4290b9457e98f/third_party/WebKit/LayoutTests/platform/linux/virtual/rootlayerscrolls/scrollbars/disabled-scrollbar-expected.png
[delete] https://crrev.com/b650c38e592cc27ed3572a8ef6d4290b9457e98f/third_party/WebKit/LayoutTests/platform/linux/virtual/rootlayerscrolls/scrollbars/resize-scales-with-dpi-150-expected.png
[delete] https://crrev.com/b650c38e592cc27ed3572a8ef6d4290b9457e98f/third_party/WebKit/LayoutTests/platform/linux/virtual/rootlayerscrolls/scrollbars/scrollbar-buttons-expected.png
[delete] https://crrev.com/b650c38e592cc27ed3572a8ef6d4290b9457e98f/third_party/WebKit/LayoutTests/platform/linux/virtual/rootlayerscrolls/scrollbars/scrollbar-orientation-expected.png
[delete] https://crrev.com/b650c38e592cc27ed3572a8ef6d4290b9457e98f/third_party/WebKit/LayoutTests/platform/linux/virtual/rootlayerscrolls/scrollbars/scrollbars-on-positioned-content-expected.png
[delete] https://crrev.com/b650c38e592cc27ed3572a8ef6d4290b9457e98f/third_party/WebKit/LayoutTests/platform/mac-mac10.9/virtual/rootlayerscrolls/scrollbars/border-box-rect-clips-scrollbars-expected.png
[delete] https://crrev.com/b650c38e592cc27ed3572a8ef6d4290b9457e98f/third_party/WebKit/LayoutTests/platform/mac-mac10.9/virtual/rootlayerscrolls/scrollbars/resize-scales-with-dpi-150-expected.png
[delete] https://crrev.com/b650c38e592cc27ed3572a8ef6d4290b9457e98f/third_party/WebKit/LayoutTests/platform/mac-mac10.9/virtual/rootlayerscrolls/scrollbars/scrollbars-on-positioned-content-expected.png
[delete] https://crrev.com/b650c38e592cc27ed3572a8ef6d4290b9457e98f/third_party/WebKit/LayoutTests/platform/mac/virtual/rootlayerscrolls/scrollbars/border-box-rect-clips-scrollbars-expected.png
[delete] https://crrev.com/b650c38e592cc27ed3572a8ef6d4290b9457e98f/third_party/WebKit/LayoutTests/platform/mac/virtual/rootlayerscrolls/scrollbars/custom-scrollbar-appearance-property-expected.png
[delete] https://crrev.com/b650c38e592cc27ed3572a8ef6d4290b9457e98f/third_party/WebKit/LayoutTests/platform/mac/virtual/rootlayerscrolls/scrollbars/resize-scales-with-dpi-150-expected.png
[delete] https://crrev.com/b650c38e592cc27ed3572a8ef6d4290b9457e98f/third_party/WebKit/LayoutTests/platform/mac/virtual/rootlayerscrolls/scrollbars/scrollbars-on-positioned-content-expected.png
[delete] https://crrev.com/b650c38e592cc27ed3572a8ef6d4290b9457e98f/third_party/WebKit/LayoutTests/platform/win/virtual/rootlayerscrolls/scrollbars/border-box-rect-clips-scrollbars-expected.png
[delete] https://crrev.com/b650c38e592cc27ed3572a8ef6d4290b9457e98f/third_party/WebKit/LayoutTests/platform/win/virtual/rootlayerscrolls/scrollbars/custom-scrollbar-appearance-property-expected.png
[delete] https://crrev.com/b650c38e592cc27ed3572a8ef6d4290b9457e98f/third_party/WebKit/LayoutTests/platform/win/virtual/rootlayerscrolls/scrollbars/scrollbars-on-positioned-content-expected.png
[modify] https://crrev.com/9e0775c12ad128945061bcd5bf1265ab1e90b4d3/third_party/WebKit/Source/core/paint/PaintLayer.cpp
[modify] https://crrev.com/9e0775c12ad128945061bcd5bf1265ab1e90b4d3/third_party/WebKit/Source/core/paint/PaintLayerPainterTest.cpp
[modify] https://crrev.com/9e0775c12ad128945061bcd5bf1265ab1e90b4d3/third_party/WebKit/Source/core/paint/ViewPainter.cpp

FYI: Filed issue 745946 to make it a little harder to unknowingly break subpixel AA