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/960937Reviewed-by: Scott Violet <sky@chromium.org>
Commit-Queue: Scott Violet <sky@chromium.org>
Cr-Commit-Position: refs/heads/master@{#542844}

testing/variations/fieldtrial_testing_config.json

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