Field trial for OptimizedRealtimeThreadingMac experiment

Bug: 1132225
Change-Id: I48fbea94bcaf267e778fc787729ed5b3e20d73c3
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2468096
Auto-Submit: Olga Sharonova <olka@chromium.org>
Commit-Queue: Brian White <bcwhite@chromium.org>
Reviewed-by: Brian White <bcwhite@chromium.org>
Cr-Commit-Position: refs/heads/master@{#816742}

testing/variations/fieldtrial_testing_config.json

@@ -4982,6 +4982,21 @@
             ]
         }
     ],
+    "OptimizedRealtimeThreadingMac": [
+        {
+            "platforms": [
+                "mac"
+            ],
+            "experiments": [
+                {
+                    "name": "Enabled",
+                    "enable_features": [
+                        "OptimizedRealtimeThreadingMac"
+                    ]
+                }
+            ]
+        }
+    ],
     "OutOfBlinkCors": [
         {
             "platforms": [