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

ios/web/web_state/ui/crw_wk_script_message_router.mm

 // Copyright 2014 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #import "ios/web/web_state/ui/crw_wk_script_message_router.h"
 
 #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.
 - (void)tryRemoveScriptMessageHandlerForName:(NSString*)messageName
                                      webView:(WKWebView*)webView;
 
 @end
 
 @implementation CRWWKScriptMessageRouter {
   // Two level map of registed message handlers. Keys are message names and
@@ skipping 13 matching lines @@
 - (instancetype)init {
   NOTREACHED();
   return nil;
 }
 
 - (instancetype)initWithUserContentController:
     (WKUserContentController*)userContentController {
   DCHECK(userContentController);
   if ((self = [super init])) {
     _handlers.reset([[NSMutableDictionary alloc] init]);
-    _userContentController.reset([userContentController retain]);
+    _userContentController.reset(userContentController);
   }
   return self;
 }
 
 - (void)setScriptMessageHandler:(void (^)(WKScriptMessage*))handler
                            name:(NSString*)messageName
                         webView:(WKWebView*)webView {
   DCHECK(handler);
   DCHECK(messageName);
   DCHECK(webView);
@@ skipping 42 matching lines @@
 
 #pragma mark -
 #pragma mark Implementation
 
 - (void)tryRemoveScriptMessageHandlerForName:(NSString*)messageName
                                      webView:(WKWebView*)webView {
   NSMapTable* webViewToHandlerMap = [_handlers objectForKey:messageName];
   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];
   } else {
     [webViewToHandlerMap removeObjectForKey:webView];
   }
 }
 
 @end