chrome/browser/download: Use BUILDFLAG for OS checking

Use BUILDFLAG(IS_XXX) instead of defined(OS_XXX).

Generated by `os_buildflag_migration.py` (https://crrev.com/c/3311983).

[email protected]

Bug: 1234043
Test: No functionality change
Change-Id: Ic39fd5f1bb4bb09af9b853b2795ffb01dd81589c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3374250
Reviewed-by: Nico Weber <[email protected]>
Commit-Queue: Xiaohan Wang <[email protected]>
Cr-Commit-Position: refs/heads/main@{#958213}
diff --git a/chrome/browser/download/download_permission_request.cc b/chrome/browser/download/download_permission_request.cc
index 95def5be..1fdbc1c 100644
--- a/chrome/browser/download/download_permission_request.cc
+++ b/chrome/browser/download/download_permission_request.cc
@@ -9,7 +9,7 @@
 #include "components/permissions/request_type.h"
 #include "ui/base/l10n/l10n_util.h"
 
-#if defined(OS_ANDROID)
+#if BUILDFLAG(IS_ANDROID)
 #include "chrome/browser/android/android_theme_resources.h"
 #include "components/url_formatter/elide_url.h"
 #include "url/origin.h"