[iOS] Small comment fix

Adding // namespace xxx after namespace definition.
This is simplify a little:
https://chromium-review.googlesource.com/c/chromium/src/+/1280249/68/ios/chrome/browser/signin/gaia_auth_fetcher_ios.h

Change-Id: Icd3b07d450ed2af65662587359bf036966f5f1f4
Reviewed-on: https://chromium-review.googlesource.com/c/1355100Reviewed-by: Mihai Sardarescu <msarda@chromium.org>
Commit-Queue: Jérôme Lebel <jlebel@chromium.org>
Cr-Commit-Position: refs/heads/master@{#612204}

ios/chrome/browser/signin/gaia_auth_fetcher_ios.h

@@ -17,15 +17,15 @@ class GURL;
 
 namespace net {
 class URLRequestStatus;
-}
+}  // namespace net
 
 namespace network {
 class SharedURLLoaderFactory;
-}
+}  // namespace network
 
 namespace web {
 class BrowserState;
-}
+}  // namespace web
 
 // Specialization of GaiaAuthFetcher on iOS.
 //