[ios] Adds a buildflag for the ios_app_bundle_id_prefix.

ios/chrome/common/app_group/app_group_constants.mm

Index: ios/chrome/common/app_group/app_group_constants.mm
diff --git a/ios/chrome/common/app_group/app_group_constants.mm b/ios/chrome/common/app_group/app_group_constants.mm
index e5911ba6de99fbd1f7fcd63c9801388a5724d279..f080bb9f6c59c935c7cc9346c563b906f7fcf773 100644
--- a/ios/chrome/common/app_group/app_group_constants.mm
+++ b/ios/chrome/common/app_group/app_group_constants.mm
@@ -7,16 +7,12 @@
 #include "base/logging.h"
 #include "base/strings/sys_string_conversions.h"
 #include "components/version_info/version_info.h"
+#include "ios/chrome/common/ios_app_bundle_id_prefix.h"
 
 #if !defined(__has_feature) || !__has_feature(objc_arc)
 #error "This file requires ARC support."
 #endif
 
-namespace {
-NSString* const kChromeAppGroupIdentifier =
-    @"group." IOS_BUNDLE_ID_PREFIX ".chrome";
-}
-
 namespace app_group {
 
 const char kChromeAppGroupXCallbackCommand[] = "app-group-command";
@@ -51,7 +47,8 @@ NSString* ApplicationGroup() {
   NSBundle* bundle = [NSBundle mainBundle];
   NSString* group = [bundle objectForInfoDictionaryKey:@"KSApplicationGroup"];
   if (![group length]) {
-    return kChromeAppGroupIdentifier;
+    return [NSString stringWithFormat:@"group.%s.chrome",
+                                      BUILDFLAG(IOS_APP_BUNDLE_ID_PREFIX), nil];
   }
   return group;
 }