Make client tag IDs consistent in sync fake server

sync/test/fake_server/fake_server_entity.cc

Index: sync/test/fake_server/fake_server_entity.cc
diff --git a/sync/test/fake_server/fake_server_entity.cc b/sync/test/fake_server/fake_server_entity.cc
index 602a266c394e60e273ca304aa52a674b1b4470a4..f3fc604fa838dfbab62d39535f84f41a28d62517 100644
--- a/sync/test/fake_server/fake_server_entity.cc
+++ b/sync/test/fake_server/fake_server_entity.cc
@@ -27,7 +27,11 @@ using std::vector;
 using syncer::ModelType;
 
 // The separator used when formatting IDs.
-const char kIdSeparator[] = "/";
+//
+// We chose the underscore character because it doesn't conflict with the
+// special characters used by base/base64.h's encoding, which is also used in
+// the construction of some IDs.
+const char kIdSeparator[] = "_";
 
 namespace fake_server {