[Ext Registry] Do not try to re-watch registry if we are already watching it.

chrome/test/BUILD.gn

Index: chrome/test/BUILD.gn
diff --git a/chrome/test/BUILD.gn b/chrome/test/BUILD.gn
index e1f2c4217bfc16f33af8b13aaa302121795f299a..a7be8ec62d3a3aef1c8f79bbf536e73ba8bbbecc 100644
--- a/chrome/test/BUILD.gn
+++ b/chrome/test/BUILD.gn
@@ -4171,6 +4171,10 @@ test("unit_tests") {
         "../browser/extensions/api/mdns/mdns_api_unittest.cc",
       ]
     }
+    if (is_win) {
+      sources +=
+          [ "../browser/extensions/external_registry_loader_win_unittest.cc" ]
+    }
     if (!is_chromeos) {
       sources += [
         "../browser/extensions/api/messaging/native_message_process_host_unittest.cc",