Add helper function to test that a given URL is a valid Gaia sign in URL.

google_apis/gaia/gaia_auth_util.h

Index: google_apis/gaia/gaia_auth_util.h
diff --git a/google_apis/gaia/gaia_auth_util.h b/google_apis/gaia/gaia_auth_util.h
index 792a59b05423a36ad5c841789d88110dfd49448e..848228c4d91dbe4d46b6450e6d75678c1b0a1426 100644
--- a/google_apis/gaia/gaia_auth_util.h
+++ b/google_apis/gaia/gaia_auth_util.h
@@ -7,6 +7,8 @@
 
 #include <string>
 
+class GURL;
+
 namespace gaia {
 
 // Perform basic canonicalization of |email_address|, taking into account that
@@ -26,6 +28,8 @@ std::string SanitizeEmail(const std::string& email_address);
 // Extract the domain part from the canonical form of the given email.
 std::string ExtractDomainName(const std::string& email);
 
+bool IsGaiaSignonRealm(const GURL& url);
+
 }  // namespace gaia
 
 #endif  // GOOGLE_APIS_GAIA_GAIA_AUTH_UTIL_H_