[Files ranking] Add a field trial testing config.

Bug: 959679
Change-Id: Ibe31eb89715b60313fef5b6772d9a1c5f2bd3c7b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1866090Reviewed-by: Nik Bhagat <nikunjb@chromium.org>
Commit-Queue: Tony Yeoman <tby@chromium.org>
Cr-Commit-Position: refs/heads/master@{#708056}

testing/variations/fieldtrial_testing_config.json

@@ -598,6 +598,21 @@
             ]
         }
     ],
+    "AppListZeroStateMixedTypesRanker": [
+        {
+            "platforms": [
+                "chromeos"
+            ],
+            "experiments": [
+                {
+                    "name": "AppListZeroStateMixedTypesRanker",
+                    "enable_features": [
+                        "EnableZeroStateMixedTypesRanker"
+                    ]
+                }
+            ]
+        }
+    ],
     "AppleScriptExecuteJavaScript": [
         {
             "platforms": [