Change GetAccessToken APIs so that they don't pass const ref to blocks.

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

Index: ios/public/provider/chrome/browser/signin/chrome_identity_service.h
diff --git a/ios/public/provider/chrome/browser/signin/chrome_identity_service.h b/ios/public/provider/chrome/browser/signin/chrome_identity_service.h
index 17054c7fd08c92fd032fd458ccabe8b88d3840d8..7d1d0cdabbca2403793d3001041e49c945562dac 100644
--- a/ios/public/provider/chrome/browser/signin/chrome_identity_service.h
+++ b/ios/public/provider/chrome/browser/signin/chrome_identity_service.h
@@ -157,14 +157,14 @@ class ChromeIdentityService {
   // Uses the default client id and client secret.
   virtual void GetAccessToken(ChromeIdentity* identity,
                               const std::set<std::string>& scopes,
-                              const AccessTokenCallback& callback);
+                              AccessTokenCallback callback);
 
   // Asynchronously retrieves access tokens for the given identity and scopes.
   virtual void GetAccessToken(ChromeIdentity* identity,
                               const std::string& client_id,
                               const std::string& client_secret,
                               const std::set<std::string>& scopes,
-                              const AccessTokenCallback& callback);
+                              AccessTokenCallback callback);
 
   // Fetches the profile avatar, from the cache or the network.
   // For high resolution iPads, returns large images (200 x 200) to avoid