[iOS] Preparing to remove client secret

This patch is to fix crrev.com/c/973303. The new method for GetAccessToken
(without the secret client) needs to be the main method. And the old
method needs to have a the fallback to the new one if it is still called.

Related to crrev.com/i/480972 and crrev.com/c/973229.

Bug: 516021
Cq-Include-Trybots: master.tryserver.chromium.mac:ios-simulator-cronet;master.tryserver.chromium.mac:ios-simulator-full-configs
Change-Id: I5086c2e26f8456a8a741072be2f46e0f90f8f91b
Reviewed-on: https://chromium-review.googlesource.com/975127Reviewed-by: Mihai Sardarescu <msarda@chromium.org>
Commit-Queue: Jérôme Lebel <jlebel@chromium.org>
Cr-Commit-Position: refs/heads/master@{#545053}

ios/public/provider/chrome/browser/signin/chrome_identity_service.mm

@@ -89,14 +89,14 @@ void ChromeIdentityService::GetAccessToken(ChromeIdentity* identity,
                                            const std::string& client_id,
                                            const std::string& client_secret,
                                            const std::set<std::string>& scopes,
-                                           AccessTokenCallback callback) {}
+                                           AccessTokenCallback callback) {
+  GetAccessToken(identity, client_id, scopes, callback);
+}
 
 void ChromeIdentityService::GetAccessToken(ChromeIdentity* identity,
                                            const std::string& client_id,
                                            const std::set<std::string>& scopes,
-                                           AccessTokenCallback callback) {
-  GetAccessToken(identity, client_id, "", scopes, callback);
-}
+                                           AccessTokenCallback callback) {}
 
 void ChromeIdentityService::GetAvatarForIdentity(ChromeIdentity* identity,
                                                  GetAvatarCallback callback) {}