[Extensions] Make Event::restrict_to_browser_context const.

chrome/browser/extensions/api/preference/preference_helpers.cc

Index: chrome/browser/extensions/api/preference/preference_helpers.cc
diff --git a/chrome/browser/extensions/api/preference/preference_helpers.cc b/chrome/browser/extensions/api/preference/preference_helpers.cc
index 2361270a861fefb22b6664582764d022cafc7b28..5d4c1296ae51c291e6598ce655e7ad8ad7227b80 100644
--- a/chrome/browser/extensions/api/preference/preference_helpers.cc
+++ b/chrome/browser/extensions/api/preference/preference_helpers.cc
@@ -128,9 +128,9 @@ void DispatchEventToExtensions(Profile* profile,
       }
 
       std::unique_ptr<base::ListValue> args_copy(args->DeepCopy());
-      std::unique_ptr<Event> event(
-          new Event(histogram_value, event_name, std::move(args_copy)));
-      event->restrict_to_browser_context = restrict_to_profile;
+      auto event =
+          base::MakeUnique<Event>(histogram_value, event_name,
+                                  std::move(args_copy), restrict_to_profile);
       router->DispatchEventToExtension(extension->id(), std::move(event));
     }
   }