[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, 2017/06/02 21:42:12]

Would like to discuss this! I'm not sure quite what the issue is here that we'd
need to do this, and  [[x retain] autorelease] isn't something we can directly
bring to ARC.

It seems the author wanted to keep this object alive for a notification. Is the
original concern that removing the |handler| from |_handlers| would deallocate
it and then it would not hear the notification that it had been removed?

If so, we could consider a new strong local variable to |handler| and make sure
it existed until the method returns. E.g.:

id local_handler = handler;
<remove object for key>
local_handler; // ensure ARC does not release the object until after this point
return

Or alternatively, keep it alive for one more runloop like:

dispatch_async(dispatch_get_main_queue(), ^{
  handler;
}

Both of these are gross, but not too much worse than the original?

[PL, 2017/06/02 21:47:45]

AHA! I think this is a candidate for NS_VALID_UNTIL_END_OF_SCOPE.

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