Mark a service test that seems flaky on Windows

runtime/observatory/tests/service/service.status

Index: runtime/observatory/tests/service/service.status
diff --git a/runtime/observatory/tests/service/service.status b/runtime/observatory/tests/service/service.status
index 182e71d0f5d1c3d963bed591780c2642571f712f..4f929c10f16f3d5d2e7e146ea4ba867dc1148750 100644
--- a/runtime/observatory/tests/service/service.status
+++ b/runtime/observatory/tests/service/service.status
@@ -80,6 +80,9 @@ dev_fs_weird_char_test: Skip # Windows disallows question mark in paths
 dev_fs_http_put_weird_char_test: Skip # Windows disallows carriage returns in paths
 complex_reload_test: Skip # Issue 27861
 
+# Flaky test, only observed on Windows so far.
+vm_timeline_flags_test: Pass, RuntimeError # Issue 28625
+
 # Service protocol is not supported in product mode.
 [ $mode == product ]
 *: SkipByDesign