Waterfall to use run-time flag WebRTC-H264WithOpenH264FFmpeg

testing/variations/fieldtrial_testing_config_mac.json

 {
     "AutofillClassifier": [
         {
             "group_name": "Enabled"
         }
     ],
     "AutofillFieldMetadata": [
         {
             "group_name": "Enabled"
         }
@@ skipping 211 matching lines @@
             "params": {
                 "fetch_period_min": "30"
             }
         }
     ],
     "WebFontsIntervention": [
         {
             "group_name": "Enabled"
         }
     ],
+    "WebRTC-H264WithOpenH264FFmpeg": [
+        {
+            "enable_features": [
+                "WebRTC-H264WithOpenH264FFmpeg"
+            ],
+            "group_name": "Enabled"
+        }
+    ],
     "WebRTC-LocalIPPermissionCheck": [
         {
             "group_name": "Enabled"
         }
     ],
     "WebRTC-PeerConnectionDTLS1.2": [
         {
             "group_name": "Enabled"
         }
     ]
 }