wangxianzhu can you help us fix this? :)

I am not sure if this should be a blocker for RLS launch or if it is just a test fix?

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/93de7f3a6651079001872768654238bc8e7b4dde

commit 93de7f3a6651079001872768654238bc8e7b4dde
Author: Chris Harrelson <chrishtr@chromium.org>
Date: Fri Feb 09 05:19:55 2018

[RLS] Apply correct paint offset root for elements that are fixed to the root.

fixed_position_children_fixed_to_root is used to set the paint_offset_root
to a fixed-position element which is not composited but whose transform
is above the scroll of the viewport. This was broken in RLS.

Bug:  793051 
Cq-Include-Trybots: master.tryserver.blink:linux_trusty_blink_rel;master.tryserver.chromium.linux:linux_layout_tests_slimming_paint_v2
Change-Id: Ie4f2a56726ca66663c9bfd1a4a11149385bbf633
Reviewed-on: https://chromium-review.googlesource.com/910075
Reviewed-by: Xianzhu Wang <wangxianzhu@chromium.org>
Commit-Queue: Xianzhu Wang <wangxianzhu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#535647}
[modify] https://crrev.com/93de7f3a6651079001872768654238bc8e7b4dde/third_party/WebKit/Source/core/paint/PaintPropertyTreeBuilder.cpp
[modify] https://crrev.com/93de7f3a6651079001872768654238bc8e7b4dde/third_party/WebKit/Source/core/paint/PaintPropertyTreeBuilderTest.cpp