sky says on https://chromium-review.googlesource.com/c/chromium/src/+/1184115

"I suspect this should be !IsUsingWindowService. The reason for that is TabScrubber is watching for gestures on the shell. I suspect that won't work quite right in single process mash either (because the gestures are actually generated in the browser side). It may just work if we switch it to use Env::GetInstance()->AddPretargetHandler()."

Based on conversation with James, I think for single process mash we should have TabScrubber as a pre-target-handler for both Shell *and* Env. This way it'll pick up scroll gestures not over a browser window.

Ryan has a fix out that trivially addresses single-process-mash. Retargetting this to multi-process-mash.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/ea3bb790e350169ca2bfb439607b89deff5cd689

commit ea3bb790e350169ca2bfb439607b89deff5cd689
Author: Ryan Cui <rcui@chromium.org>
Date: Thu Aug 30 21:30:49 2018

Enable tab scrubber for SingleProcessMash

Verified functionality on an Eve.  This will need to be revisited for
Mash.

BUG: 796366
Change-Id: I7b32728ff2440a8f5cda9b5ac0c28ad30182b145
Reviewed-on: https://chromium-review.googlesource.com/1197367
Reviewed-by: Scott Violet <sky@chromium.org>
Commit-Queue: Ryan Cui <rcui@chromium.org>
Cr-Commit-Position: refs/heads/master@{#587777}
[modify] https://crrev.com/ea3bb790e350169ca2bfb439607b89deff5cd689/chrome/browser/ui/ash/chrome_browser_main_extra_parts_ash.cc