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

BUG=704551
TEST=ios_chrome_unittests

Review-Url: https://codereview.chromium.org/2967273002
Cr-Commit-Position: refs/heads/master@{#484727}
diff --git a/ios/chrome/browser/BUILD.gn b/ios/chrome/browser/BUILD.gn
index 56163da..e9bafc0 100644
--- a/ios/chrome/browser/BUILD.gn
+++ b/ios/chrome/browser/BUILD.gn
@@ -94,6 +94,7 @@
     "//components/net_log",
     "//components/ntp_tiles",
     "//components/password_manager/core/common:common",
+    "//components/payments/core",
     "//components/pref_registry",
     "//components/prefs",
     "//components/proxy_config",