[Extensions] Make Event::restrict_to_browser_context const.

chrome/browser/speech/extension_api/tts_extension_api.cc

Index: chrome/browser/speech/extension_api/tts_extension_api.cc
diff --git a/chrome/browser/speech/extension_api/tts_extension_api.cc b/chrome/browser/speech/extension_api/tts_extension_api.cc
index d8de59d92123386dd222f946f5869ac932db42a0..4ea4334ae7576c9d71f5ac076a39e70b94b5a941 100644
--- a/chrome/browser/speech/extension_api/tts_extension_api.cc
+++ b/chrome/browser/speech/extension_api/tts_extension_api.cc
@@ -140,10 +140,9 @@ void TtsExtensionEventHandler::OnTtsEvent(Utterance* utterance,
   std::unique_ptr<base::ListValue> arguments(new base::ListValue());
   arguments->Append(std::move(details));
 
-  std::unique_ptr<extensions::Event> event(
-      new extensions::Event(::extensions::events::TTS_ON_EVENT,
-                            ::events::kOnEvent, std::move(arguments)));
-  event->restrict_to_browser_context = utterance->browser_context();
+  auto event = base::MakeUnique<extensions::Event>(
+      ::extensions::events::TTS_ON_EVENT, ::events::kOnEvent,
+      std::move(arguments), utterance->browser_context());
   event->event_url = utterance->src_url();
   extensions::EventRouter::Get(utterance->browser_context())
       ->DispatchEventToExtension(src_extension_id_, std::move(event));