[iOS] Support empty callbacks in all CookieStoreIOS methods.

This CL adds the missing tests for empty callbacks, to match the other
CookieStore implementations.
Callers do expect empty callbacks to be supported, resulting in actual
crashes. This CL fixes these crashes.

BUG=587930

Review URL: https://codereview.chromium.org/1717253002

Cr-Commit-Position: refs/heads/master@{#376771}

ios/net/cookies/cookie_store_ios.mm

@@ -1081,7 +1081,8 @@ bool HasExplicitDomain(const std::string& cookie_line) {
   CookieStoreIOSClient* client = net::GetCookieStoreIOSClient();
   DCHECK(client);
   client->DidChangeCookieStorage();
-  callback.Run(num_deleted_from_nshttp_cookie_storage);
+  if (!callback.is_null())
+    callback.Run(num_deleted_from_nshttp_cookie_storage);
 }
 
 void CookieStoreIOS::UpdateCachesFromCookieMonster() {
@@ -1099,20 +1100,23 @@ bool HasExplicitDomain(const std::string& cookie_line) {
   DCHECK(thread_checker_.CalledOnValidThread());
   if (success)
     UpdateCachesFromCookieMonster();
-  callback.Run(success);
+  if (!callback.is_null())
+    callback.Run(success);
 }
 
 void CookieStoreIOS::UpdateCachesAfterDelete(const DeleteCallback& callback,
                                              int num_deleted) {
   DCHECK(thread_checker_.CalledOnValidThread());
   UpdateCachesFromCookieMonster();
-  callback.Run(num_deleted);
+  if (!callback.is_null())
+    callback.Run(num_deleted);
 }
 
 void CookieStoreIOS::UpdateCachesAfterClosure(const base::Closure& callback) {
   DCHECK(thread_checker_.CalledOnValidThread());
   UpdateCachesFromCookieMonster();
-  callback.Run();
+  if (!callback.is_null())
+    callback.Run();
 }
 
 net::CookieList