Replace remaining defined(OS_FOO) with BUILDFLAG(IS_FOO).

For all IS_FOO build flags defined aside from IS_WIN and IS_APPLE.

Bug: chromium:1234043
Change-Id: I61b4d4e98c335052604ff514faeac50a6fb2948a
Reviewed-on: https://pdfium-review.googlesource.com/c/pdfium/+/89231
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 91aacab..5823ee1 100644
--- a/fxjs/cjs_util.cpp
+++ b/fxjs/cjs_util.cpp
@@ -27,7 +27,7 @@
 #include "third_party/base/cxx17_backports.h"
 #include "v8/include/v8-date.h"
 
-#if defined(OS_ANDROID)
+#if BUILDFLAG(IS_ANDROID)
 #include <ctype.h>
 #endif