Replace defined(OS_WIN) with BUILDFLAG(IS_WIN).

Bug: chromium:1234043
Change-Id: I5d2bccc92b5d3d3417d73377f3457646b95be90f
Reviewed-on: https://pdfium-review.googlesource.com/c/pdfium/+/89052
Reviewed-by: Tom Sepez <tsepez@chromium.org>
Commit-Queue: Lei Zhang <thestig@chromium.org>
diff --git a/fxjs/cjs_util.cpp b/fxjs/cjs_util.cpp
index 11820a6..91aacab 100644
--- a/fxjs/cjs_util.cpp
+++ b/fxjs/cjs_util.cpp
@@ -52,7 +52,7 @@
     {L"ddd", L"%a"},  {L"dd", L"%d"},  {L"yyyy", L"%Y"}, {L"yy", L"%y"},
     {L"HH", L"%H"},   {L"hh", L"%I"},  {L"MM", L"%M"},   {L"ss", L"%S"},
     {L"TT", L"%p"},
-#if defined(OS_WIN)
+#if BUILDFLAG(IS_WIN)
     {L"tt", L"%p"},   {L"h", L"%#I"},
 #else
     {L"tt", L"%P"},   {L"h", L"%l"},