Since AnimationWorklet is an experimental feature, I propose that we should not block BGPT. I will fix this but wanted to make sure this is clear. :)

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/726e68d05c3db9d60cbfe889fa2541456a81a6a9

commit 726e68d05c3db9d60cbfe889fa2541456a81a6a9
Author: Xianzhu Wang <wangxianzhu@chromium.org>
Date: Fri Sep 14 18:46:33 2018

[BGPT] Update FlagExpectations/enable-blink-gen-property-trees

TBR=pdr@chromium.org

Bug: 884239, 884240 
Cq-Include-Trybots: luci.chromium.try:linux-blink-gen-property-trees;luci.chromium.try:linux_layout_tests_slimming_paint_v2;master.tryserver.blink:linux_trusty_blink_rel
Change-Id: Iafece0358cf595c3055c481ac7af7368dde110f7
Reviewed-on: https://chromium-review.googlesource.com/1226354
Commit-Queue: Xianzhu Wang <wangxianzhu@chromium.org>
Reviewed-by: Xianzhu Wang <wangxianzhu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#591413}
[modify] https://crrev.com/726e68d05c3db9d60cbfe889fa2541456a81a6a9/third_party/WebKit/LayoutTests/FlagExpectations/enable-blink-gen-property-trees

 Issue 878514  has been merged into this issue.

AFAICT this test is no longer failing on ToT. Will try to run a reverse bisect to see if I can find how it got fixed. 

./tools/bisect-builds.py --verify-range -a linux64 -b 591413 -g 624822 --use-local-cache --  --no-first-run --user-data-dir=/tmp --enable-blink-features=AnimationWorklet,WebAnimationsAPI --enable-blink-gen-property-trees http://localhost:8000/worklet-animation-local-time-undefined.html

This reduced the range to:a change made after 593261 (known bad), but no later than 593267 (first known good).
CHANGELOG URL:
  https://chromium.googlesource.com/chromium/src/+log/41bdcc1802a1cb5b8233f2fae57156add132eee1..e94fb27bfaa6483f49464abe07b41caf2f748964

But there is no CL that makes sense there. So I suspect the test may actually be flaky and need more investigation :(