Fix build with enable_downgrade_processing=false

Setting GN flag `enable_downgrade_processing` to false fails to build
in desktop platforms. Use the correct BUILDFLAG when calling
`downgrade::RegisterPrefs`.

Change-Id: I34d0c2b59f88b07478a3f28cdf9b2371d7522be0
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6190700
Reviewed-by: Gabriel Charette <[email protected]>
Commit-Queue: Gabriel Charette <[email protected]>
Cr-Commit-Position: refs/heads/main@{#1411157}
diff --git a/chrome/browser/prefs/browser_prefs.cc b/chrome/browser/prefs/browser_prefs.cc
index bc75bdd8..b97cb4f 100644
--- a/chrome/browser/prefs/browser_prefs.cc
+++ b/chrome/browser/prefs/browser_prefs.cc
@@ -514,7 +514,7 @@
 #include "chrome/browser/ui/startup/first_run_service.h"
 #endif
 
-#if !BUILDFLAG(IS_ANDROID) && !BUILDFLAG(IS_CHROMEOS)
+#if BUILDFLAG(ENABLE_DOWNGRADE_PROCESSING)
 #include "chrome/browser/downgrade/downgrade_prefs.h"
 #endif
 
@@ -1873,7 +1873,7 @@
 #endif  // BUILDFLAG(GOOGLE_CHROME_BRANDING)
 #endif  // BUILDFLAG(IS_WIN)
 
-#if !BUILDFLAG(IS_ANDROID) && !BUILDFLAG(IS_CHROMEOS)
+#if BUILDFLAG(ENABLE_DOWNGRADE_PROCESSING)
   downgrade::RegisterPrefs(registry);
 #endif