Add CloudPolicyOverFcm in field_trial_testing_config

The experiment CloudPolicyOverFCM was tested on 50% Dev and Canary and
the result were expected. Prior to moving to 50% Beta the experiment
is added to Field Trial Testing Config.

The results of experiment:
* https://screenshot.googleplex.com/6ZNs6sQ7QxM

Bug: 939039
Change-Id: Id8d7107a93d64a93f3b8363909fd691af517cab6
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1906215Reviewed-by: Steven Holte <holte@chromium.org>
Commit-Queue: Askar Aitzhan <askaraitzhan@google.com>
Cr-Commit-Position: refs/heads/master@{#714478}

testing/variations/fieldtrial_testing_config.json

@@ -1585,6 +1585,21 @@
             ]
         }
     ],
+    "CloudPolicyOverFcm": [
+        {
+            "platforms": [
+                "chromeos",
+                "linux",
+                "mac",
+                "windows"
+            ],
+            "experiments": [
+                {
+                    "name": "CloudPolicyOverFCM"
+                }
+            ]
+        }
+    ],
     "CompositorImageAnimation": [
         {
             "platforms": [