I think this affects the following tests:

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
compositing/iframes/connect-compositing-iframe2.html
compositing/iframes/connect-compositing-iframe3.html
compositing/iframes/enter-compositing-iframe.html
compositing/iframes/iframe-resize.html
compositing/iframes/iframe-size-from-zero.html
compositing/iframes/invisible-nested-iframe-show.html
compositing/iframes/overlapped-iframe.html
compositing/iframes/overlapped-iframe-iframe.html
compositing/iframes/resizer.html
compositing/iframes/scrolling-iframe.html
compositing/rtl/rtl-iframe-absolute.html
compositing/rtl/rtl-iframe-absolute-overflow.html
compositing/rtl/rtl-iframe-absolute-overflow-scrolled.html
compositing/rtl/rtl-iframe-fixed.html
compositing/rtl/rtl-iframe-fixed-overflow.html
compositing/rtl/rtl-iframe-fixed-overflow-scrolled.html
compositing/rtl/rtl-iframe-relative.html

The red flag is "Child Containment Layer" in RLS tree (where non-RLS didn't have it).

Note: I think issue 782991 would fix this, but I'm going to see if there's an easier way.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/157d2b153e3992fac6dcc5c1eaba428a6d823814

commit 157d2b153e3992fac6dcc5c1eaba428a6d823814
Author: Steve Kobes <skobes@chromium.org>
Date: Mon Nov 13 19:06:08 2017

RLS: Enable composited scrolling in composited iframes.

This fixes 16 of the 21 compositing/ layout tests listed on  crbug.com/778469 .
The remaining 5 are affected by  crbug.com/784053 .

This patch includes new baselines for the tests that are fixed (diff against
generic baselines in patchset 1).  These reflect the presence of scrolling
layers in the layer tree dump, and "flattenInheritedTransform": false due to
crbug.com/778060.

Bug:  778469 
Cq-Include-Trybots: master.tryserver.chromium.linux:linux_layout_tests_slimming_paint_v2
Change-Id: I1cffa503561942aef7c932f6d52ae541dc88e8a2
Reviewed-on: https://chromium-review.googlesource.com/764952
Reviewed-by: Philip Rogers <pdr@chromium.org>
Commit-Queue: Steve Kobes <skobes@chromium.org>
Cr-Commit-Position: refs/heads/master@{#515995}
[modify] https://crrev.com/157d2b153e3992fac6dcc5c1eaba428a6d823814/third_party/WebKit/LayoutTests/FlagExpectations/root-layer-scrolls
[add] https://crrev.com/157d2b153e3992fac6dcc5c1eaba428a6d823814/third_party/WebKit/LayoutTests/flag-specific/root-layer-scrolls/compositing/iframes/become-overlapped-iframe-expected.txt
[add] https://crrev.com/157d2b153e3992fac6dcc5c1eaba428a6d823814/third_party/WebKit/LayoutTests/flag-specific/root-layer-scrolls/compositing/iframes/composited-parent-iframe-expected.txt
[add] https://crrev.com/157d2b153e3992fac6dcc5c1eaba428a6d823814/third_party/WebKit/LayoutTests/flag-specific/root-layer-scrolls/compositing/iframes/connect-compositing-iframe-delayed-expected.txt
[add] https://crrev.com/157d2b153e3992fac6dcc5c1eaba428a6d823814/third_party/WebKit/LayoutTests/flag-specific/root-layer-scrolls/compositing/iframes/connect-compositing-iframe-expected.txt
[add] https://crrev.com/157d2b153e3992fac6dcc5c1eaba428a6d823814/third_party/WebKit/LayoutTests/flag-specific/root-layer-scrolls/compositing/iframes/connect-compositing-iframe2-expected.txt
[add] https://crrev.com/157d2b153e3992fac6dcc5c1eaba428a6d823814/third_party/WebKit/LayoutTests/flag-specific/root-layer-scrolls/compositing/iframes/connect-compositing-iframe3-expected.txt
[add] https://crrev.com/157d2b153e3992fac6dcc5c1eaba428a6d823814/third_party/WebKit/LayoutTests/flag-specific/root-layer-scrolls/compositing/iframes/enter-compositing-iframe-expected.txt
[add] https://crrev.com/157d2b153e3992fac6dcc5c1eaba428a6d823814/third_party/WebKit/LayoutTests/flag-specific/root-layer-scrolls/compositing/iframes/iframe-resize-expected.txt
[add] https://crrev.com/157d2b153e3992fac6dcc5c1eaba428a6d823814/third_party/WebKit/LayoutTests/flag-specific/root-layer-scrolls/compositing/iframes/iframe-size-from-zero-expected.txt
[add] https://crrev.com/157d2b153e3992fac6dcc5c1eaba428a6d823814/third_party/WebKit/LayoutTests/flag-specific/root-layer-scrolls/compositing/iframes/overlapped-iframe-expected.txt
[add] https://crrev.com/157d2b153e3992fac6dcc5c1eaba428a6d823814/third_party/WebKit/LayoutTests/flag-specific/root-layer-scrolls/compositing/iframes/resizer-expected.txt
[add] https://crrev.com/157d2b153e3992fac6dcc5c1eaba428a6d823814/third_party/WebKit/LayoutTests/flag-specific/root-layer-scrolls/compositing/iframes/scrolling-iframe-expected.txt
[add] https://crrev.com/157d2b153e3992fac6dcc5c1eaba428a6d823814/third_party/WebKit/LayoutTests/flag-specific/root-layer-scrolls/compositing/rtl/rtl-iframe-absolute-overflow-expected.txt
[add] https://crrev.com/157d2b153e3992fac6dcc5c1eaba428a6d823814/third_party/WebKit/LayoutTests/flag-specific/root-layer-scrolls/compositing/rtl/rtl-iframe-absolute-overflow-scrolled-expected.txt
[add] https://crrev.com/157d2b153e3992fac6dcc5c1eaba428a6d823814/third_party/WebKit/LayoutTests/flag-specific/root-layer-scrolls/compositing/rtl/rtl-iframe-fixed-overflow-expected.txt
[add] https://crrev.com/157d2b153e3992fac6dcc5c1eaba428a6d823814/third_party/WebKit/LayoutTests/flag-specific/root-layer-scrolls/compositing/rtl/rtl-iframe-fixed-overflow-scrolled-expected.txt
[modify] https://crrev.com/157d2b153e3992fac6dcc5c1eaba428a6d823814/third_party/WebKit/Source/core/paint/compositing/CompositingRequirementsUpdater.cpp
[modify] https://crrev.com/157d2b153e3992fac6dcc5c1eaba428a6d823814/third_party/WebKit/Source/core/paint/compositing/CompositingRequirementsUpdater.h