ListAccounts will return the Gaia ID as well as the email of the account.

google_apis/gaia/fake_gaia.cc

Index: google_apis/gaia/fake_gaia.cc
diff --git a/google_apis/gaia/fake_gaia.cc b/google_apis/gaia/fake_gaia.cc
index 88fb274f452d7ad9ce3f7cf2f982bc840c8ce885..456d8db13e91763c9b57e2381371c6a739b3d325 100644
--- a/google_apis/gaia/fake_gaia.cc
+++ b/google_apis/gaia/fake_gaia.cc
@@ -51,7 +51,7 @@ const char kTestOAuthLoginSID[] = "fake-oauth-SID-cookie";
 const char kTestOAuthLoginLSID[] = "fake-oauth-LSID-cookie";
 const char kTestOAuthLoginAuthCode[] = "fake-oauth-auth-code";
 
-const char kDefaultGaiaId[]  ="12345";
+const char kDefaultGaiaId[] = "12345";
 
 const base::FilePath::CharType kServiceLogin[] =
     FILE_PATH_LITERAL("google_apis/test/service_login.html");
@@ -64,7 +64,7 @@ const char kAuthHeaderBearer[] = "Bearer ";
 const char kAuthHeaderOAuth[] = "OAuth ";
 
 const char kListAccountsResponseFormat[] =
-    "[\"gaia.l.a.r\",[[\"gaia.l.a\",1,\"\",\"%s\",\"\",1,1,0]]]";
+    "[\"gaia.l.a.r\",[[\"gaia.l.a\",1,\"\",\"%s\",\"\",1,1,0,0,1,\"12345\"]]]";
 
 typedef std::map<std::string, std::string> CookieMap;