The above line is wrong, should be:

third_party/blink/tools/run_web_tests.py -t ChromeDebug --additional-driver-flag=--enable-blink-gen-property-trees virtual/threaded/synthetic_gestures/synthetic-pinch-zoom-gesture-touchpad.html --skipped=ignore

Still repros but passes without BGPT.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/adc4e84b405bdf17be6f602c450586e5a5a9a8ab

commit adc4e84b405bdf17be6f602c450586e5a5a9a8ab
Author: chaopeng <chaopeng@chromium.org>
Date: Fri Sep 07 17:22:12 2018

Call rAFs before checking the pinch zoom result

This test failing because it need some rAFs for commit.

Bug:  875097 
Change-Id: I10aacf4a55ea8fe277fef1fc68b4d6c9ee287190
Reviewed-on: https://chromium-review.googlesource.com/1213545
Reviewed-by: David Bokan <bokan@chromium.org>
Commit-Queue: Jianpeng Chao <chaopeng@chromium.org>
Cr-Commit-Position: refs/heads/master@{#589562}
[modify] https://crrev.com/adc4e84b405bdf17be6f602c450586e5a5a9a8ab/third_party/WebKit/LayoutTests/virtual/threaded/synthetic_gestures/synthetic-pinch-zoom-gesture.js

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/4cc4e51514fb31f92aa4e8c84a182f68e074beb8

commit 4cc4e51514fb31f92aa4e8c84a182f68e074beb8
Author: David Bokan <bokan@chromium.org>
Date: Fri Sep 14 19:09:23 2018

[blink-gen-property-trees] Remove passing expectation

This test was fixed and is passing as of
https://crrev.com/c/adc4e84b405bdf17be6f602c450586e5a5a9a8ab

Bug:  875097 
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: I60e11d3a79f087eb255e4144e08f4eb9caa362a8
Reviewed-on: https://chromium-review.googlesource.com/1227145
Reviewed-by: Jianpeng Chao <chaopeng@chromium.org>
Commit-Queue: David Bokan <bokan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#591422}
[modify] https://crrev.com/4cc4e51514fb31f92aa4e8c84a182f68e074beb8/third_party/WebKit/LayoutTests/FlagExpectations/enable-blink-gen-property-trees