Tests that produce a layer tree dump (those that output internal.layerTreeAsText(document)) will need new baselines for root layer scrolling.  Many of those tests were identified and rebaselined using an automated tool relying regexp matching:

https://chromium-review.googlesource.com/699752

However, the regexp matching was not sophisticated enough to identify tests with iframes that can be rebaselined.  Those tests need to be manually inspected to determine whether the new layer tree dump is correct; and if so, rebaselined.

Here are a few sample failing tests (there are a bunch more):

compositing/fixed-body-background-positioned.html
compositing/force-compositing-mode/overflow-iframe-enter-compositing.html
compositing/force-compositing-mode/overflow-iframe-layer.html
compositing/iframes/become-composited-nested-iframes.html
compositing/iframes/become-overlapped-iframe.html
compositing/iframes/composited-parent-iframe.html
compositing/iframes/connect-compositing-iframe-delayed.html
compositing/iframes/connect-compositing-iframe.html