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

ios/web/web_state/web_state_observer_bridge.mm

 // Copyright 2015 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/public/web_state/web_state_observer_bridge.h"
 
+#if !defined(__has_feature) || !__has_feature(objc_arc)
+#error "This file requires ARC support."
+#endif
+
 namespace web {
 
 WebStateObserverBridge::WebStateObserverBridge(web::WebState* webState,
                                                id<CRWWebStateObserver> observer)
     : web::WebStateObserver(webState), observer_(observer) {
 }
 
 WebStateObserverBridge::~WebStateObserverBridge() {
 }
 
@@ skipping 98 matching lines @@
 void WebStateObserverBridge::WebStateDestroyed() {
   SEL selector = @selector(webStateDestroyed:);
   if ([observer_ respondsToSelector:selector]) {
     // |webStateDestroyed:| may delete |this|, so don't expect |this| to be
     // valid afterwards.
     [observer_ webStateDestroyed:web_state()];
   }
 }
 
 }  // namespace web