reland attempt 4: Adds WindowService-ChromeOS fieldtrial

More bugs fixed. Landing again.

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
TBR=rkaplow@chromium.org

Change-Id: I474b97bc4e8d5904aa07b8de67e1b961adbac1f7
Reviewed-on: https://chromium-review.googlesource.com/970913Reviewed-by: Scott Violet <sky@chromium.org>
Commit-Queue: Scott Violet <sky@chromium.org>
Cr-Commit-Position: refs/heads/master@{#544415}

testing/variations/fieldtrial_testing_config.json

@@ -4291,6 +4291,21 @@
             ]
         }
     ],
+    "WindowService-ChromeOS": [
+        {
+            "platforms": [
+                "chromeos"
+            ],
+            "experiments": [
+                {
+                    "name": "Enabled",
+                    "enable_features": [
+                        "Mus"
+                    ]
+                }
+            ]
+        }
+    ],
     "WorkStealingInScriptRunner": [
         {
             "platforms": [