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/core/fxge/cfx_fontmapper.h b/core/fxge/cfx_fontmapper.h
index 956232e..183e59c 100644
--- a/core/fxge/cfx_fontmapper.h
+++ b/core/fxge/cfx_fontmapper.h
@@ -75,12 +75,12 @@
   bool HasInstalledFont(ByteStringView name) const;
   bool HasLocalizedFont(ByteStringView name) const;
 
-#if defined(OS_WIN)
+#if BUILDFLAG(IS_WIN)
   absl::optional<ByteString> InstalledFontNameStartingWith(
       const ByteString& name) const;
   absl::optional<ByteString> LocalizedFontNameStartingWith(
       const ByteString& name) const;
-#endif  // defined(OS_WIN)
+#endif  // BUILDFLAG(IS_WIN)
 
 #ifdef PDF_ENABLE_XFA
   std::unique_ptr<uint8_t, FxFreeDeleter> RawBytesForIndex(