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}

testing/variations/fieldtrial_testing_config.json

@@ -4290,6 +4290,21 @@
             ]
         }
     ],
+    "WindowService-ChromeOS": [
+        {
+            "platforms": [
+                "chromeos"
+            ],
+            "experiments": [
+                {
+                    "name": "Enabled",
+                    "enable_features": [
+                        "Mus"
+                    ]
+                }
+            ]
+        }
+    ],
     "WorkStealingInScriptRunner": [
         {
             "platforms": [