Found via bisect. See https://bugs.chromium.org/p/chromium/issues/detail?id=821592

Revert "Reland "Adds WindowService-ChromeOS fieldtrial""

This reverts commit 299e1847.

Reason for revert: <INSERT REASONING HERE>

Original change's description:
> Reland "Adds WindowService-ChromeOS fieldtrial"
> 
> This reverts commit 6c097bb5.
> 
> Reason for revert: I'm reverting the revert as the dependent change has cycled through the bots https://chromium.googlesource.com/chromium/src/+/a5ea6086a54429ad2770cf28a3898413866e1bdd
> 
> Original change's description:
> > Revert "Adds WindowService-ChromeOS fieldtrial"
> > 
> > This reverts commit b4603176.
> > 
> > Reason for revert: Reverting as one of the fixs for an interactive_ui_test failure got reverted.
> > 
> > Original change's description:
> > > Adds WindowService-ChromeOS fieldtrial
> > > 
> > > I'm not sure on the name here. I went with WindowService-ChromeOS as
> > > at some point we will launch the WindowService on other platforms, but
> > > that won't be anytime soon and is likely to be very different. So, I'm
> > > thinking later on we have the field trial WindowService-Desktop.
> > > 
> > > BUG=814898
> > > TEST=none
> > > 
> > > Change-Id: I5bf45c2a463de0fefaeb06939429977e36c3681e
> > > Reviewed-on: https://chromium-review.googlesource.com/932821
> > > Commit-Queue: Scott Violet <sky@chromium.org>
> > > Reviewed-by: Robert Kaplow <rkaplow@chromium.org>
> > > Cr-Commit-Position: refs/heads/master@{#542680}
> > 
> > TBR=sky@chromium.org,rkaplow@chromium.org
> > 
> > Change-Id: Id63c00a90c6fd05ff74f0ffaa6ed738a98bd74e3
> > No-Presubmit: true
> > No-Tree-Checks: true
> > No-Try: true
> > Bug: 814898
> > Reviewed-on: https://chromium-review.googlesource.com/959763
> > Reviewed-by: Scott Violet <sky@chromium.org>
> > Commit-Queue: Scott Violet <sky@chromium.org>
> > Cr-Commit-Position: refs/heads/master@{#542713}
> 
> TBR=sky@chromium.org,rkaplow@chromium.org
> 
> Change-Id: I5a7259e6d6ce0ddafb516c22f847e6f662ad57b5
> No-Presubmit: true
> No-Tree-Checks: true
> No-Try: true
> Bug: 814898
> Reviewed-on: https://chromium-review.googlesource.com/960937
> Reviewed-by: Scott Violet <sky@chromium.org>
> Commit-Queue: Scott Violet <sky@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#542844}

TBR=sky@chromium.org,rkaplow@chromium.org

Change-Id: If4b14ec456142c815d6caec76551672e99729348
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: 814898
Reviewed-on: https://chromium-review.googlesource.com/961323Reviewed-by: David Tseng <dtseng@chromium.org>
Commit-Queue: David Tseng <dtseng@chromium.org>
Cr-Commit-Position: refs/heads/master@{#542935}

testing/variations/fieldtrial_testing_config.json

@@ -4284,21 +4284,6 @@
             ]
         }
     ],
-    "WindowService-ChromeOS": [
-        {
-            "platforms": [
-                "chromeos"
-            ],
-            "experiments": [
-                {
-                    "name": "Enabled",
-                    "enable_features": [
-                        "Mus"
-                    ]
-                }
-            ]
-        }
-    ],
     "WorkStealingInScriptRunner": [
         {
             "platforms": [