Switch to standard integer types in ui/.

BUG=138542
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Review URL: https://codereview.chromium.org/1550483002

Cr-Commit-Position: refs/heads/master@{#366842}
diff --git a/ui/accessibility/ax_text_utils_unittest.cc b/ui/accessibility/ax_text_utils_unittest.cc
index 806401b..1c2a4a1e 100644
--- a/ui/accessibility/ax_text_utils_unittest.cc
+++ b/ui/accessibility/ax_text_utils_unittest.cc
@@ -2,6 +2,8 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+#include <stddef.h>
+
 #include "base/strings/utf_string_conversions.h"
 #include "testing/gtest/include/gtest/gtest.h"
 #include "ui/accessibility/ax_text_utils.h"