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

ios/chrome/browser/web/chrome_web_client.mm

 // Copyright 2014 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #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"
 #include "components/version_info/version_info.h"
 #include "ios/chrome/browser/application_context.h"
 #include "ios/chrome/browser/browser_about_rewriter.h"
 #include "ios/chrome/browser/browser_state/chrome_browser_state.h"
 #include "ios/chrome/browser/chrome_switches.h"
 #include "ios/chrome/browser/chrome_url_constants.h"
 #include "ios/chrome/browser/experimental_flags.h"
@@ skipping 134 matching lines @@
 
 void ChromeWebClient::PostBrowserURLRewriterCreation(
     web::BrowserURLRewriter* rewriter) {
   rewriter->AddURLRewriter(&WillHandleWebBrowserAboutURL);
 }
 
 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 = @"%@; %@";
   return [NSString stringWithFormat:kScriptTemplate,
                                     GetPageScript(@"payment_request"),
                                     chrome_page_script];
 }
 
 void ChromeWebClient::AllowCertificateError(
     web::WebState* web_state,
     int cert_error,
     const net::SSLInfo& info,
     const GURL& request_url,
     bool overridable,
     const base::Callback<void(bool)>& callback) {
   IOSSSLErrorHandler::HandleSSLError(web_state, cert_error, info, request_url,
                                      overridable, callback);
 }
 
 std::unique_ptr<base::TaskScheduler::InitParams>
 ChromeWebClient::GetTaskSchedulerInitParams() {
   return task_scheduler_util::GetBrowserTaskSchedulerInitParamsFromVariations();
 }
 
 bool ChromeWebClient::IsSlimNavigationManagerEnabled() const {
   return experimental_flags::IsSlimNavigationManagerEnabled();
 }