[ObjC ARC] Converts parts of ios/web/webstate to ARC.

ios/web/web_state/ui/crw_wk_script_message_router.mm

Index: ios/web/web_state/ui/crw_wk_script_message_router.mm
diff --git a/ios/web/web_state/ui/crw_wk_script_message_router.mm b/ios/web/web_state/ui/crw_wk_script_message_router.mm
index 0e1dfbb33e0dd333119542563b4cbd246663c9b7..0c041ac21c10eeb9d454f8c2b8a4c2d9cbf99252 100644
--- a/ios/web/web_state/ui/crw_wk_script_message_router.mm
+++ b/ios/web/web_state/ui/crw_wk_script_message_router.mm
@@ -7,6 +7,10 @@
 #import "base/logging.h"
 #import "base/mac/scoped_nsobject.h"
 
+#if !defined(__has_feature) || !__has_feature(objc_arc)
+#error "This file requires ARC support."
+#endif
+
 @interface CRWWKScriptMessageRouter ()<WKScriptMessageHandler>
 
 // Removes a specific message handler. Does nothing if handler does not exist.
@@ -40,7 +44,7 @@
   DCHECK(userContentController);
   if ((self = [super init])) {
     _handlers.reset([[NSMutableDictionary alloc] init]);
-    _userContentController.reset([userContentController retain]);
+    _userContentController.reset(userContentController);
   }
   return self;
 }
@@ -103,9 +107,6 @@
   id handler = [webViewToHandlerMap objectForKey:webView];
   if (!handler)
     return;
-  // Extend the lifetime of |handler| so removeScriptMessageHandlerForName: can
-  // be called from inside of |handler|.
-  [[handler retain] autorelease];
   if (webViewToHandlerMap.count == 1) {
     [_handlers removeObjectForKey:messageName];
     [_userContentController removeScriptMessageHandlerForName:messageName];