I think we may be hitting this in https://codereview.chromium.org/2531603003.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/9bf0adfb29b2f4c92c658a31d5562cded817746b

commit 9bf0adfb29b2f4c92c658a31d5562cded817746b
Author: pdr <pdr@chromium.org>
Date: Sat Jan 14 00:50:32 2017

Implement Internals::mainThreadScrollingReasons for SPV2

This patch returns FrameView's main thread scrolling reasons
as a string which is used by Internals::mainThreadScrollingReasons.

TEST=compositing/layer-creation/iframe-background-attachment-fixed.html with
--additional-driver-flag=--enable-slimming-paint-v2

BUG= 671838 

CQ_INCLUDE_TRYBOTS=master.tryserver.chromium.linux:linux_layout_tests_slimming_paint_v2

Review-Url: https://codereview.chromium.org/2634493006
Cr-Commit-Position: refs/heads/master@{#443744}

[modify] https://crrev.com/9bf0adfb29b2f4c92c658a31d5562cded817746b/third_party/WebKit/LayoutTests/FlagExpectations/enable-slimming-paint-v2
[modify] https://crrev.com/9bf0adfb29b2f4c92c658a31d5562cded817746b/third_party/WebKit/Source/core/frame/FrameView.cpp

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/9bf0adfb29b2f4c92c658a31d5562cded817746b

commit 9bf0adfb29b2f4c92c658a31d5562cded817746b
Author: pdr <pdr@chromium.org>
Date: Sat Jan 14 00:50:32 2017

Implement Internals::mainThreadScrollingReasons for SPV2

This patch returns FrameView's main thread scrolling reasons
as a string which is used by Internals::mainThreadScrollingReasons.

TEST=compositing/layer-creation/iframe-background-attachment-fixed.html with
--additional-driver-flag=--enable-slimming-paint-v2

BUG= 671838 

CQ_INCLUDE_TRYBOTS=master.tryserver.chromium.linux:linux_layout_tests_slimming_paint_v2

Review-Url: https://codereview.chromium.org/2634493006
Cr-Commit-Position: refs/heads/master@{#443744}

[modify] https://crrev.com/9bf0adfb29b2f4c92c658a31d5562cded817746b/third_party/WebKit/LayoutTests/FlagExpectations/enable-slimming-paint-v2
[modify] https://crrev.com/9bf0adfb29b2f4c92c658a31d5562cded817746b/third_party/WebKit/Source/core/frame/FrameView.cpp