[ObjC ARC] Converts ios/web:web to ARC.

ios/web/web_state/web_state_impl.mm

Index: ios/web/web_state/web_state_impl.mm
diff --git a/ios/web/web_state/web_state_impl.mm b/ios/web/web_state/web_state_impl.mm
index de5e4b97b79629f782af4f2a69367a7974b32070..7808f7502e6668ff4ab93ee8c2cc04b3616e1f86 100644
--- a/ios/web/web_state/web_state_impl.mm
+++ b/ios/web/web_state/web_state_impl.mm
@@ -38,6 +38,10 @@
 #include "ios/web/webui/web_ui_ios_impl.h"
 #include "net/http/http_response_headers.h"
 
+#if !defined(__has_feature) || !__has_feature(objc_arc)
+#error "This file requires ARC support."
+#endif
+
 namespace web {
 
 /* static */
@@ -162,7 +166,7 @@ CRWWebController* WebStateImpl::GetWebController() {
 
 void WebStateImpl::SetWebController(CRWWebController* web_controller) {
   [web_controller_ close];
-  web_controller_.reset([web_controller retain]);
+  web_controller_.reset(web_controller);
 }
 
 void WebStateImpl::OnTitleChanged() {