[ObjC ARC] Converts ios/chrome/browser/prefs:browser_prefs to ARC.

Automatically generated ARCMigrate commit
Notable issues:None
BUG=624363
TEST=None

Review-Url: https://codereview.chromium.org/2893603003
Cr-Commit-Position: refs/heads/master@{#475854}

ios/chrome/browser/prefs/BUILD.gn

@@ -24,6 +24,7 @@ source_set("prefs") {
 }
 
 source_set("browser_prefs") {
+  configs += [ "//build/config/compiler:enable_arc" ]
   sources = [
     "browser_prefs.h",
     "browser_prefs.mm",

ios/chrome/browser/prefs/browser_prefs.mm

@@ -54,6 +54,10 @@
 #include "ios/public/provider/chrome/browser/chrome_browser_provider.h"
 #include "ui/base/l10n/l10n_util.h"
 
+#if !defined(__has_feature) || !__has_feature(objc_arc)
+#error "This file requires ARC support."
+#endif
+
 void RegisterLocalStatePrefs(PrefRegistrySimple* registry) {
   BrowserStateInfoCache::RegisterPrefs(registry);
   flags_ui::PrefServiceFlagsStorage::RegisterPrefs(registry);