reland attempt 3: 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
TBR=rkaplow@chromium.org

Change-Id: I555c9cea0f59c28b2e6124196f362ac3231104fc
Reviewed-on: https://chromium-review.googlesource.com/962877Reviewed-by: Scott Violet <sky@chromium.org>
Commit-Queue: Scott Violet <sky@chromium.org>
Cr-Commit-Position: refs/heads/master@{#543156}

testing/variations/fieldtrial_testing_config.json

@@ -4300,6 +4300,21 @@
             ]
         }
     ],
+    "WindowService-ChromeOS": [
+        {
+            "platforms": [
+                "chromeos"
+            ],
+            "experiments": [
+                {
+                    "name": "Enabled",
+                    "enable_features": [
+                        "Mus"
+                    ]
+                }
+            ]
+        }
+    ],
     "WorkStealingInScriptRunner": [
         {
             "platforms": [