[Payment Request] window.PaymentRequest must be undefined if PR is disabled

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 0c9883ba549a932dfa5661242790d7fc7a65a401..f8abb2c695ff05a619a8f6962b6c233be6940e1e 100644
--- a/ios/chrome/browser/web/chrome_web_client.mm
+++ b/ios/chrome/browser/web/chrome_web_client.mm
@@ -161,7 +161,15 @@ void ChromeWebClient::PostBrowserURLRewriterCreation(
 
 NSString* ChromeWebClient::GetEarlyPageScript(
     web::BrowserState* browser_state) const {
-  return GetPageScript(@"chrome_bundle");
+  NSString* chrome_page_script = GetPageScript(@"chrome_bundle");
+
+  if (!experimental_flags::IsPaymentRequestEnabled())
+    return chrome_page_script;
+
+  NSString* kScriptTemplate = @"%@; %@";
+  return [NSString stringWithFormat:kScriptTemplate,
+                                    GetPageScript(@"payment_request"),
+                                    chrome_page_script];
 }
 
 void ChromeWebClient::AllowCertificateError(