https://github.com/chromium/chromium/commit/574a7eb053a0ca3ae94872a0cac72e0ba76ecd6d

--- a/third_party/blink/renderer/core/html/forms/select_mutation_observer.h
+++ b/third_party/blink/renderer/core/html/forms/select_mutation_observer.h
@@ -2,6 +2,9 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+#ifndef THIRD_PARTY_BLINK_RENDERER_CORE_HTML_FORMS_SELECT_MUTATION_OBSERVER_H_
+#define THIRD_PARTY_BLINK_RENDERER_CORE_HTML_FORMS_SELECT_MUTATION_OBSERVER_H_
+
 #include "third_party/blink/renderer/core/dom/mutation_observer.h"
 #include "third_party/blink/renderer/core/html/forms/html_select_element.h"
 #include "third_party/blink/renderer/core/inspector/inspector_audits_issue.h"
@@ -56,3 +59,5 @@ class SelectMutationObserver : public MutationObserver::Delegate {
 };
 
 }  // namespace blink
+
+#endif  // THIRD_PARTY_BLINK_RENDERER_CORE_HTML_FORMS_SELECT_MUTATION_OBSERVER_H_