Add WebRTC-Vp9ExternalRefCtrl finch experiment to testing config

Bug: 1027108
Change-Id: Idb394958879058c2e05da571ca4ba5f684ddd1aa
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2060976
Commit-Queue: Ilya Nikolaevskiy <ilnik@chromium.org>
Commit-Queue: Robert Kaplow <rkaplow@chromium.org>
Auto-Submit: Ilya Nikolaevskiy <ilnik@chromium.org>
Reviewed-by: Robert Kaplow <rkaplow@chromium.org>
Cr-Commit-Position: refs/heads/master@{#742150}

testing/variations/fieldtrial_testing_config.json

@@ -7978,6 +7978,23 @@
             ]
         }
     ],
+    "WebRTC-Vp9ExternalRefCtrl": [
+        {
+            "platforms": [
+                "windows",
+                "mac",
+                "chromeos",
+                "linux",
+                "ios",
+                "android"
+            ],
+            "experiments": [
+                {
+                    "name": "Enabled"
+                }
+            ]
+        }
+    ],
     "WebRtcHybridAgc": [
         {
             "platforms": [