Pref service: enable for user prefs in chrome behind a flag.

chrome/browser/chrome_content_browser_manifest_overlay.json

 {
   "name": "content_browser",
   "display_name": "Chrome",
   "interface_provider_specs": {
     "service_manager:connector": {
       "provides": {
         "renderer": [
           "autofill::mojom::AutofillDriver",
           "autofill::mojom::PasswordManagerDriver",
           "chrome::mojom::NetBenchmarking",
           "extensions::StashService",
           "metrics::mojom::LeakDetector",
           "mojom::ModuleEventSink",
           "rappor::mojom::RapporRecorder",
           "startup_metric_utils::mojom::StartupMetricHost",
           "translate::mojom::ContentTranslateDriver"
         ],
         "gpu": [
           "metrics::mojom::CallStackProfileCollector"
         ],
         "ime:ime_driver": []
       },
       "requires": {
         "accessibility_autoclick": [ "ash:autoclick" ],
         "ash": [ "ash" ],
         "image_decoder": [ "decode" ],
         "nacl_broker": [ "browser" ],
         "nacl_loader": [ "browser" ],
         "preferences": [ "preferences_manager" ],
-        "preferences2": [ "pref_client" ],
+        "preferences2": [ "pref_client", "pref_control" ],
         "ui": [ "display_controller", "ime_registrar" ]
       }
     },
     "navigation:frame": {
       "provides": {
         "renderer": [
           "autofill::mojom::AutofillDriver",
           "autofill::mojom::PasswordManagerDriver",
           "blink::mojom::InstalledAppProvider",
           "blink::mojom::ShareService",
@@ skipping 13 matching lines @@
           // TODO(beng): These should be moved to a separate capability.
           "mojom::OmniboxPageHandler",
           "mojom::PluginsPageHandler",
           "mojom::SiteEngagementUIHandler",
           "mojom::UsbInternalsPageHandler"
         ]
       }
     }
   }
 }