[Cast Streaming] Add MediaVideoEncoder fieldtrial config

This patch adds a fieldtrial testing configuration for the
CastStreamingMediaVideoEncoder experiment.

Bug: 282984511
Change-Id: I18ecefcf3cecf89ba90f4de88801a567d2861d8b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6354280
Commit-Queue: Jordan Bayles <jophba@chromium.org>
Reviewed-by: Ben Pastene <bpastene@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1432456}

testing/variations/fieldtrial_testing_config.json

@@ -4054,6 +4054,24 @@
             ]
         }
     ],
+    "CastStreamingMediaVideoEncoder": [
+        {
+            "platforms": [
+                "chromeos",
+                "linux",
+                "mac",
+                "windows"
+            ],
+            "experiments": [
+                {
+                    "name": "Enabled",
+                    "enable_features": [
+                        "CastStreamingMediaVideoEncoder"
+                    ]
+                }
+            ]
+        }
+    ],
     "CastStreamingVp9": [
         {
             "platforms": [