Enable field trial testing config for AVFoundationCaptureV2.

Bug: 1127791
Change-Id: I9e73069d8cf7a296f2929d325aafed28e8c42962
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2407899Reviewed-by: Dale Curtis <dalecurtis@chromium.org>
Reviewed-by: Jesse Doherty <jwd@chromium.org>
Commit-Queue: Markus Handell <handellm@google.com>
Cr-Commit-Position: refs/heads/master@{#808049}

testing/variations/fieldtrial_testing_config.json

 {
+    "AVFoundationCaptureV2": [
+        {
+            "platforms": [
+                "mac"
+            ],
+            "experiments": [
+                {
+                    "name": "Enabled",
+                    "enable_features": [
+                        "AVFoundationCaptureV2"
+                    ]
+                }
+            ]
+        }
+    ],
     "AddToHomescreenMessaging": [
         {
             "platforms": [