[ObjC ARC] Converts ios/web:web 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 ed13ddb0b878def502fbd03555fc35f32ee2a154..8aa0f7067675220e6038afdbc42efd0fbdd3406e 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
@@ -5,7 +5,10 @@
 #import "ios/web/web_state/ui/crw_wk_script_message_router.h"
 
 #include "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>
 
@@ -18,16 +21,16 @@
 @implementation CRWWKScriptMessageRouter {
   // Two level map of registed message handlers. Keys are message names and
   // values are more maps (where keys are web views and values are handlers).
-  base::scoped_nsobject<NSMutableDictionary> _handlers;
+  NSMutableDictionary* _handlers;
   // Wrapped WKUserContentController.
-  base::scoped_nsobject<WKUserContentController> _userContentController;
+  WKUserContentController* _userContentController;
 }
 
 #pragma mark -
 #pragma mark Interface
 
 - (WKUserContentController*)userContentController {
-  return _userContentController.get();
+  return _userContentController;
 }
 
 - (instancetype)init {
@@ -39,8 +42,8 @@
     (WKUserContentController*)userContentController {
   DCHECK(userContentController);
   if ((self = [super init])) {
-    _handlers.reset([[NSMutableDictionary alloc] init]);
-    _userContentController.reset([userContentController retain]);
+    _handlers = [[NSMutableDictionary alloc] init];
+    _userContentController = userContentController;
   }
   return self;
 }
@@ -103,9 +106,7 @@
   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];
+  // ARC TODO: Discuss the lifecycle of handler here in ARC.

[PL, 2017/06/01 01:56:21]

This is something that needs discussion. 

We were keeping handler alive until the end of the runloop at least here, but we
can't do anything like this in ARC. I need to dig in deeper through code
analysis to try and understand the thinking here, because the comment is a bit
cryptic to me.

Would appreciate any guidance :-).

   if (webViewToHandlerMap.count == 1) {
     [_handlers removeObjectForKey:messageName];
     [_userContentController removeScriptMessageHandlerForName:messageName];