Add MacOS to SettingsEnforcement's enabled fieldtrial testing platforms

testing/variations/fieldtrial_testing_config.json

 {
     "AlternateComponentUrls": [
         {
             "platforms": [
                 "android",
                 "chromeos",
                 "ios",
                 "linux",
                 "mac",
                 "win"
@@ skipping 2624 matching lines @@
                     "disable_features": [
                         "ServiceWorkerNavigationPreload"
                     ]
                 }
             ]
         }
     ],
     "SettingsEnforcement": [
         {
             "platforms": [
+                "mac",
                 "win"
             ],
             "experiments": [
                 {
                     "name": "enforce_always_with_extensions_and_dse"
                 }
             ]
         }
     ],
     "SettingsResetPrompt": [
@@ skipping 630 matching lines @@
                 {
                     "name": "Enabled",
                     "enable_features": [
                         "use-new-media-cache"
                     ]
                 }
             ]
         }
     ]
 }