Improve documentation of CookieNotificationBridge

The ownership of the "observer_" member field of CookieNotificationBridge
was unclear and after a casual look at the code, it was unclear whether
the code caused a retain cycle or not.

Rename the member field, mark it as __strong and add documentation to
make it clear that the ownership model is correct, and that there is no
retain cycle caused by this object.

Bug: 1005153
Change-Id: I8f49e39e8355a0a2c758848ef34c45b4d5335b66
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1809376
Auto-Submit: Sylvain Defresne <sdefresne@chromium.org>
Reviewed-by: Stepan Khapugin <stkhapugin@chromium.org>
Commit-Queue: Sylvain Defresne <sdefresne@chromium.org>
Cr-Commit-Position: refs/heads/master@{#697649}

ios/web/net/cookie_notification_bridge.h

@@ -7,6 +7,7 @@
 
 #include "base/macros.h"
 
+@protocol NSObject;
 @class NSNotification;
 
 namespace web {
@@ -21,7 +22,10 @@ class CookieNotificationBridge {
 
  private:
   static void OnNotificationReceived(NSNotification* notification);
-  id observer_;
+
+  // Token returned by NSNotificationCenter upon registration. Owned by the
+  // bridge and used to unregister from NSNotificationCenter in destructor.
+  __strong id<NSObject> registration_;
 
   DISALLOW_COPY_AND_ASSIGN(CookieNotificationBridge);
 };

ios/web/net/cookie_notification_bridge.mm

@@ -20,18 +20,18 @@
 namespace web {
 
 CookieNotificationBridge::CookieNotificationBridge() {
-  id<NSObject> observer = [[NSNotificationCenter defaultCenter]
+  id<NSObject> registration = [[NSNotificationCenter defaultCenter]
       addObserverForName:NSHTTPCookieManagerCookiesChangedNotification
                   object:[NSHTTPCookieStorage sharedHTTPCookieStorage]
                    queue:nil
               usingBlock:^(NSNotification* notification) {
                 OnNotificationReceived(notification);
               }];
-  observer_ = observer;
+  registration_ = registration;
 }
 
 CookieNotificationBridge::~CookieNotificationBridge() {
-  [[NSNotificationCenter defaultCenter] removeObserver:observer_];
+  [[NSNotificationCenter defaultCenter] removeObserver:registration_];
 }
 
 void CookieNotificationBridge::OnNotificationReceived(