[ObjC ARC] Converts the rest of ios/web to ARC.

ios/web/web_state/web_state_observer.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.
 
 #include "ios/web/public/web_state/web_state_observer.h"
 
 #include "ios/web/public/load_committed_details.h"
 #include "ios/web/public/web_state/web_state.h"
 
+#if !defined(__has_feature) || !__has_feature(objc_arc)
+#error "This file requires ARC support."
+#endif
+
 namespace web {
 
 int WebStateObserver::kInvalidFormKeyCode = -1;
 
 WebStateObserver::WebStateObserver(WebState* web_state) : web_state_(nullptr) {
   Observe(web_state);
 }
 
 WebStateObserver::WebStateObserver() : web_state_(nullptr) {}
 
@@ skipping 13 matching lines @@
   if (web_state_)
     web_state_->AddObserver(this);
 }
 
 void WebStateObserver::ResetWebState() {
   web_state_->RemoveObserver(this);
   web_state_ = nullptr;
 }
 
 }  // namespace web