Add missing include guard to testing_cursor_client_observer.h

Change-Id: I4ff4225a9601a8fce52cf59e6502f3ed3e0cfa28
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1982552Reviewed-by: ccameron <ccameron@chromium.org>
Commit-Queue: Henrique Ferreiro <hferreiro@igalia.com>
Cr-Commit-Position: refs/heads/master@{#727570}

ui/wm/test/testing_cursor_client_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 UI_WM_TEST_TESTING_CURSOR_CLIENT_OBSERVER_H_
+#define UI_WM_TEST_TESTING_CURSOR_CLIENT_OBSERVER_H_
+
 #include "ui/aura/client/cursor_client_observer.h"
 #include "ui/wm/core/cursor_manager.h"
 
@@ -32,3 +35,5 @@ class TestingCursorClientObserver : public aura::client::CursorClientObserver {
 };
 
 }  // namespace wm
+
+#endif  // UI_WM_TEST_TESTING_CURSOR_CLIENT_OBSERVER_H_