[ObjC ARC] Converts ios/chrome/browser/signin:signin to ARC.

ios/chrome/browser/signin/chrome_identity_service_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.
 
 #include "ios/chrome/browser/signin/chrome_identity_service_observer_bridge.h"
 
 #include "base/logging.h"
 #include "ios/public/provider/chrome/browser/chrome_browser_provider.h"
 
+#if !defined(__has_feature) || !__has_feature(objc_arc)
+#error "This file requires ARC support."
+#endif
+
 ChromeIdentityServiceObserverBridge::ChromeIdentityServiceObserverBridge(
     id<ChromeIdentityServiceObserver> observer)
     : observer_(observer), scoped_observer_(this) {
   DCHECK(observer_);
   scoped_observer_.Add(
       ios::GetChromeBrowserProvider()->GetChromeIdentityService());
 }
 
 ChromeIdentityServiceObserverBridge::~ChromeIdentityServiceObserverBridge() {}
 
@@ skipping 18 matching lines @@
 }
 
 void ChromeIdentityServiceObserverBridge::
     OnChromeIdentityServiceWillBeDestroyed() {
   if ([observer_
           respondsToSelector:@selector(
                                  onChromeIdentityServiceWillBeDestroyed)]) {
     [observer_ onChromeIdentityServiceWillBeDestroyed];
   }
 }