Add OverlayPrioritizationTesting to the field trial config

Bug: 1113306
Change-Id: I631f1d722acf8c39c296643e16c12ce6f3d0e95e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2585629Reviewed-by: Robert Kroeger <rjkroege@chromium.org>
Commit-Queue: Peter McNeeley <petermcneeley@chromium.org>
Cr-Commit-Position: refs/heads/master@{#836764}

testing/variations/fieldtrial_testing_config.json

@@ -5243,6 +5243,21 @@
             ]
         }
     ],
+    "OverlayPrioritizationTesting": [
+        {
+            "platforms": [
+                "chromeos"
+            ],
+            "experiments": [
+                {
+                    "name": "Enabled",
+                    "enable_features": [
+                        "EnableOverlayPrioritization"
+                    ]
+                }
+            ]
+        }
+    ],
     "PDFViewerUpdate": [
         {
             "platforms": [