[Autoplay] Add field trial testing config

Add field trial testing config for preseed stable exp.

BUG=931791

Change-Id: I5db4299e1a347cb52565d862a60f1b0ff3c12783
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1540458
Commit-Queue: Alexei Svitkine <asvitkine@chromium.org>
Reviewed-by: Alexei Svitkine <asvitkine@chromium.org>
Cr-Commit-Position: refs/heads/master@{#644801}

testing/variations/fieldtrial_testing_config.json

@@ -5168,6 +5168,24 @@
             ]
         }
     ],
+    "UnifiedAutoplay": [
+        {
+            "platforms": [
+                "linux",
+                "mac",
+                "chromeos",
+                "windows"
+            ],
+            "experiments": [
+                {
+                    "name": "PreseedControl",
+                    "disable_features": [
+                        "PreloadMediaEngagementData"
+                    ]
+                }
+            ]
+        }
+    ],
     "UnifiedConsent": [
         {
             "platforms": [