Here is what I have found so far:
 - First we need to make sure appropriate property nodes are creates when there is a composited animation. This is the same as  issue 776533  which the test by pass by also including "will-transform"
 - WorkletAnimation is creating appropriate CompositorAnimationPlayer but for some reason the player is not pushed to impl side! One potential is that we are not marking "SetNeedsCommit" on LTH which means the player is never pushed to impl side.

I've added virtual/threaded/fast/animationworklet/animation-worklet-scroll-timeline.html
 here as well, as it seems related. Feel free to split off into a separate bug if needs be.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/d0e536bb6e495969f6c0beda02cb9f276b988cd6

commit d0e536bb6e495969f6c0beda02cb9f276b988cd6
Author: W. James MacLean <wjmaclean@chromium.org>
Date: Fri Dec 01 16:16:49 2017

Disable flakey test on Slimming Paint V2.

virtual/threaded/fast/animationworklet/animation-worklet-scroll-timeline.html

is very broken with SPv2.

TBR=majidvp@chromium.org

Bug: 778875
Cq-Include-Trybots: master.tryserver.chromium.linux:linux_layout_tests_slimming_paint_v2
Change-Id: Ic94109ad74c09dfcbc48f4bf44a3ba134fda0918
Reviewed-on: https://chromium-review.googlesource.com/802637
Reviewed-by: James MacLean <wjmaclean@chromium.org>
Commit-Queue: James MacLean <wjmaclean@chromium.org>
Cr-Commit-Position: refs/heads/master@{#520952}
[modify] https://crrev.com/d0e536bb6e495969f6c0beda02cb9f276b988cd6/third_party/WebKit/LayoutTests/FlagExpectations/enable-slimming-paint-v2