Enable MediaFoundationH264Encoding feature

testing/variations/fieldtrial_testing_config_win.json

 {
     "AutofillClassifier": [
         {
             "group_name": "Enabled"
         }
     ],
     "AutofillFieldMetadata": [
         {
             "group_name": "Enabled"
         }
@@ skipping 179 matching lines @@
     "MaterialDesignUserManager": [
         {
             "enable_features": [
                 "MaterialDesignUserManager"
             ],
             "group_name": "Enabled"
         }
     ],
     "MediaFoundationH264Encoding": [
         {
+            "enable_features": [
+                "MediaFoundationH264Encoding"
+            ],
             "group_name": "Enabled"
         }
     ],
     "MojoChannel": [
         {
             "group_name": "Enabled"
         }
     ],
     "NetworkQualityEstimator": [
         {
@@ skipping 421 matching lines @@
     ],
     "use-new-media-cache": [
         {
             "enable_features": [
                 "use-new-media-cache"
             ],
             "group_name": "Enabled"
         }
     ]
 }