ChromeOS cryptohome should be able to use gaia id as user identifier.

components/signin/core/account_id/account_id.cc

Index: components/signin/core/account_id/account_id.cc
diff --git a/components/signin/core/account_id/account_id.cc b/components/signin/core/account_id/account_id.cc
index 09e5cdf7081212a20e795e580f3c10962f064090..5da16ae1b2c38b0c4bd3ecb624c01de4ca47a3b3 100644
--- a/components/signin/core/account_id/account_id.cc
+++ b/components/signin/core/account_id/account_id.cc
@@ -9,6 +9,7 @@
 #include "base/json/json_reader.h"
 #include "base/json/json_writer.h"
 #include "base/memory/singleton.h"
+#include "base/strings/string_util.h"
 #include "base/values.h"
 #include "google_apis/gaia/gaia_auth_util.h"
 
@@ -49,6 +50,12 @@ AccountId::AccountId() {}
 
 AccountId::AccountId(const std::string& gaia_id, const std::string& user_email)
     : gaia_id_(gaia_id), user_email_(user_email) {
+  // Fail if e-mail looks similar to GaiaIdKey.
+  LOG_ASSERT(!base::StartsWith(user_email, kKeyGaiaIdPrefix,
+                               base::CompareCase::SENSITIVE) ||
+             user_email.find('@') != std::string::npos)
+      << "Bad e-mail: '" << user_email << "' with gaia_id='" << gaia_id << "'";
+
   // TODO(alemate): DCHECK(!email.empty());
   // TODO(alemate): check gaia_id is not empty once it is required.
 }