Field trial testing config for UMA background sessions on Android

R=chromium-metrics-reviews@google.com

Bug: 954310
Change-Id: I4a612300989a1aee0ed03ab859e0a132ac91ff0b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2581518
Commit-Queue: Caitlin Fischer <caitlinfischer@google.com>
Reviewed-by: Caitlin Fischer <caitlinfischer@google.com>
Auto-Submit: Ilya Sherman <isherman@chromium.org>
Cr-Commit-Position: refs/heads/master@{#835204}

testing/variations/fieldtrial_testing_config.json

@@ -7368,6 +7368,21 @@
             ]
         }
     ],
+    "UMABackgroundSessions": [
+        {
+            "platforms": [
+                "android"
+            ],
+            "experiments": [
+                {
+                    "name": "Enabled",
+                    "enable_features": [
+                        "UMABackgroundSessions"
+                    ]
+                }
+            ]
+        }
+    ],
     "UnidoPreferSync": [
         {
             "platforms": [