[ObjC ARC] Converts components/reading_list/ios:ios to ARC.

components/reading_list/ios/reading_list_model_bridge_observer.mm

 // Copyright 2016 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 "components/reading_list/ios/reading_list_model_bridge_observer.h"
 
 #include "components/reading_list/core/reading_list_entry.h"
 #include "components/reading_list/core/reading_list_model.h"
 
+#if !defined(__has_feature) || !__has_feature(objc_arc)
+#error "This file requires ARC support."
+#endif
+
 ReadingListModelBridge::ReadingListModelBridge(
     id<ReadingListModelBridgeObserver> observer,
     ReadingListModel* model)
     : observer_(observer), model_(model) {

[noyau (Ping after 24h), 2017/05/29 11:57:59]

This now has the side effect of retaining the model, as the header only has a
comment specifying weak: With ARC this is now strong by default.

In the header model_ needs to be marked as unsafe_unretained at a minimum.

[stkhapugin, 2017/05/30 13:57:51]

In reading_list_model_bridge_observer.h, replace:
    ReadingListModel* model_;  // weak
with:
    __unsafe_unretained ReadingListModel* model_;

Ideally, try to replace it with:
    __weak   ReadingListModel* model_;  
and see if it builds. If it does, also replace: 
    __unsafe_unretained id<ReadingListModelBridgeObserver> observer_;
with:
    __weak id<ReadingListModelBridgeObserver> observer_;

[lindsayw, 2017/06/01 16:49:39]

It won't build with __weak   ReadingListModel* model_; so I put
__unsafe_unretained ReadingListModel* model_; and that also doesn't build
successfully. The error for the latter is:
../../components/reading_list/ios/reading_list_model_bridge_observer.h:78:3:
error: 'objc_ownership' only applies to Objective-C object or block pointer
types; type here is 'ReadingListModel *' [-Werror,-Wignored-attributes]

[stkhapugin, 2017/06/01 16:58:43]

Oh, disregard that then, turns out ReadingListModel is not an objc class, but a
C++ one. No need to change anything. 

[lindsayw, 2017/06/02 14:13:08]

I've added a toto to refactor the weak pointer. PTAL, noyau :)

   DCHECK(model);
   model_->AddObserver(this);
 }
 
 ReadingListModelBridge::~ReadingListModelBridge() {
   if (model_) {
     model_->RemoveObserver(this);
   }
 }
 
@@ skipping 71 matching lines @@
 }
 
 void ReadingListModelBridge::ReadingListWillUpdateEntry(
     const ReadingListModel* model,
     const GURL& url) {
   if ([observer_
           respondsToSelector:@selector(readingListModel:willUpdateEntry:)]) {
     [observer_ readingListModel:model willUpdateEntry:url];
   }
 }