[ObjC ARC] Converts ios/chrome/app:main to ARC.

ios/chrome/app/chrome_exe_main.mm

Index: ios/chrome/app/chrome_exe_main.mm
diff --git a/ios/chrome/app/chrome_exe_main.mm b/ios/chrome/app/chrome_exe_main.mm
index 52e072ca708d9588cbf7cebfd04994f87a1359d8..308e50d4abbce2fc58dccb4ca416cf179a4ca9d1 100644
--- a/ios/chrome/app/chrome_exe_main.mm
+++ b/ios/chrome/app/chrome_exe_main.mm
@@ -12,6 +12,10 @@
 #include "ios/chrome/browser/crash_report/crash_keys.h"
 #include "ios/chrome/common/channel_info.h"
 
+#if !defined(__has_feature) || !__has_feature(objc_arc)
+#error "This file requires ARC support."
+#endif
+
 namespace {
 
 NSString* const kUIApplicationDelegateInfoKey = @"UIApplicationDelegate";