One and only one top-loading-frame across all renderers.

components/test_runner/layout_test_runtime_flags.h

Index: components/test_runner/layout_test_runtime_flags.h
diff --git a/components/test_runner/layout_test_runtime_flags.h b/components/test_runner/layout_test_runtime_flags.h
index 0a4355b50abd67ce59bb9de76448024463d91912..78c8cbf895e8fde6701a3dc94522c3167d101449 100644
--- a/components/test_runner/layout_test_runtime_flags.h
+++ b/components/test_runner/layout_test_runtime_flags.h
@@ -158,6 +158,12 @@ class TEST_RUNNER_EXPORT LayoutTestRuntimeFlags {
   DEFINE_BOOL_LAYOUT_TEST_RUNTIME_FLAG(
       stay_on_page_after_handling_before_unload)
 
+  // Indicates if the test already tracks a top loading frame (in any of the
+  // renderer processes).  This flag is trying to prevent different renderer
+  // processes from tracking different top loading frames (i.e. main frame in
+  // one renderer and an OOPIF in another renderer).
+  DEFINE_BOOL_LAYOUT_TEST_RUNTIME_FLAG(have_top_loading_frame)
+
 #undef DEFINE_BOOL_LAYOUT_TEST_RUNTIME_FLAG
 #undef DEFINE_STRING_LAYOUT_TEST_RUNTIME_FLAG