Enable field trial testing config for VideoToolboxVp9Decoding.

Won't really do anything until we get macOS 11.0 bots, but this is
a requisite step for enabling on the beta channel.

Bug: 1103432
Change-Id: Ia9feafd622ce2f55c0c4905789a45c69e1e5d80a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2382220
Auto-Submit: Dale Curtis <dalecurtis@chromium.org>
Commit-Queue: Jesse Doherty <jwd@chromium.org>
Reviewed-by: Jesse Doherty <jwd@chromium.org>
Cr-Commit-Position: refs/heads/master@{#803320}

testing/variations/fieldtrial_testing_config.json

@@ -7417,6 +7417,21 @@
             ]
         }
     ],
+    "VideoToolboxVp9Decoding": [
+        {
+            "platforms": [
+                "mac"
+            ],
+            "experiments": [
+                {
+                    "name": "Enabled",
+                    "enable_features": [
+                        "VideoToolboxVp9Decoding"
+                    ]
+                }
+            ]
+        }
+    ],
     "VisualClientSideDetectionModel": [
         {
             "platforms": [