Do IWYU for <limits.h>

Add some missing STL includes. Found by building with g++.

Change-Id: I246a55e9054cdd8a402a658c9789a8a044020c7d
Reviewed-on: https://pdfium-review.googlesource.com/c/pdfium/+/121892
Reviewed-by: Tom Sepez <tsepez@chromium.org>
Reviewed-by: Tom Sepez <tsepez@google.com>
Commit-Queue: Tom Sepez <tsepez@google.com>

core/fxcrt/widestring_unittest.cpp

diff --git a/core/fxcrt/widestring_unittest.cpp b/core/fxcrt/widestring_unittest.cpp
index e18c165..a15bdc9 100644
--- a/core/fxcrt/widestring_unittest.cpp
+++ b/core/fxcrt/widestring_unittest.cpp
@@ -4,6 +4,8 @@
 
 #include "core/fxcrt/widestring.h"
 
+#include <limits.h>
+
 #include <algorithm>
 #include <iterator>
 #include <vector>

fpdfsdk/fpdf_structtree_embeddertest.cpp

diff --git a/fpdfsdk/fpdf_structtree_embeddertest.cpp b/fpdfsdk/fpdf_structtree_embeddertest.cpp
index a315bd3..f5a8b0f 100644
--- a/fpdfsdk/fpdf_structtree_embeddertest.cpp
+++ b/fpdfsdk/fpdf_structtree_embeddertest.cpp
@@ -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 <limits.h>
+
 #include <iterator>
 #include <optional>