[Extensions] Make Event::restrict_to_browser_context const.

extensions/browser/api/idle/idle_manager.cc

Index: extensions/browser/api/idle/idle_manager.cc
diff --git a/extensions/browser/api/idle/idle_manager.cc b/extensions/browser/api/idle/idle_manager.cc
index 8f4621bb3f96b897e55b685f0764b0e70f970b14..0fb5346b21cdd77bcbd8a717d4b25d22b5efe807 100644
--- a/extensions/browser/api/idle/idle_manager.cc
+++ b/extensions/browser/api/idle/idle_manager.cc
@@ -50,10 +50,9 @@ void DefaultEventDelegate::OnStateChanged(const std::string& extension_id,
                                           ui::IdleState new_state) {
   std::unique_ptr<base::ListValue> args(new base::ListValue());
   args->Append(IdleManager::CreateIdleValue(new_state));
-  std::unique_ptr<Event> event(new Event(events::IDLE_ON_STATE_CHANGED,
-                                         idle::OnStateChanged::kEventName,
-                                         std::move(args)));
-  event->restrict_to_browser_context = context_;
+  auto event = base::MakeUnique<Event>(events::IDLE_ON_STATE_CHANGED,
+                                       idle::OnStateChanged::kEventName,
+                                       std::move(args), context_);
   EventRouter::Get(context_)
       ->DispatchEventToExtension(extension_id, std::move(event));
 }