[NTPOptOut] Add testing config for NTPArticleSuggestionsExpandableHeader

Change-Id: I5e9e977d1fd072b8b7f2b3ce8193214bb6c8fe4a
Reviewed-on: https://chromium-review.googlesource.com/1060431Reviewed-by: Theresa <twellington@chromium.org>
Reviewed-by: Robert Kaplow <rkaplow@chromium.org>
Commit-Queue: Becky Zhou <huayinz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#559335}

testing/variations/fieldtrial_testing_config.json

@@ -1901,6 +1901,21 @@
             ]
         }
     ],
+    "NTPArticleSuggestionsExpandableHeader": [
+        {
+            "platforms": [
+                "android"
+            ],
+            "experiments": [
+                {
+                    "name": "Enabled",
+                    "enable_features": [
+                        "NTPArticleSuggestionsExpandableHeader"
+                    ]
+                }
+            ]
+        }
+    ],
     "NTPBreakingNewsPush": [
         {
             "platforms": [