[Payments] Add an entry for Payment Request to chrome://flags on iOS

ios/chrome/browser/web/chrome_web_client.mm

Index: ios/chrome/browser/web/chrome_web_client.mm
diff --git a/ios/chrome/browser/web/chrome_web_client.mm b/ios/chrome/browser/web/chrome_web_client.mm
index 06a3d0056a52a5854797a6718d9dd8f8df4273c5..92f5cbbd1161a81f4c8c17610682b1c550b99458 100644
--- a/ios/chrome/browser/web/chrome_web_client.mm
+++ b/ios/chrome/browser/web/chrome_web_client.mm
@@ -5,12 +5,14 @@
 #include "ios/chrome/browser/web/chrome_web_client.h"
 
 #include "base/command_line.h"
+#include "base/feature_list.h"
 #include "base/files/file_util.h"
 #include "base/ios/ios_util.h"
 #include "base/mac/bundle_locations.h"
 #include "base/memory/ptr_util.h"
 #include "base/strings/sys_string_conversions.h"
 #include "components/dom_distiller/core/url_constants.h"
+#include "components/payments/core/features.h"
 #include "components/prefs/pref_service.h"
 #include "components/strings/grit/components_strings.h"
 #include "components/task_scheduler_util/browser/initialization.h"
@@ -165,7 +167,7 @@ NSString* ChromeWebClient::GetEarlyPageScript(
     web::BrowserState* browser_state) const {
   NSString* chrome_page_script = GetPageScript(@"chrome_bundle");
 
-  if (!experimental_flags::IsPaymentRequestEnabled())
+  if (!base::FeatureList::IsEnabled(payments::features::kWebPayments))
     return chrome_page_script;
 
   NSString* kScriptTemplate = @"%@; %@";