Add New Download Manager feature to chrome://flags.

The flag is still disabled by default.

Bug: 791806
Cq-Include-Trybots: master.tryserver.chromium.mac:ios-simulator-cronet;master.tryserver.chromium.mac:ios-simulator-full-configs
Change-Id: Ibc438f98e359e7f4c1c86ce10c175e5b94344cef
Reviewed-on: https://chromium-review.googlesource.com/889642
Commit-Queue: Eugene But <eugenebut@chromium.org>
Reviewed-by: Sylvain Defresne <sdefresne@chromium.org>
Cr-Commit-Position: refs/heads/master@{#532937}

ios/chrome/browser/about_flags.mm

@@ -202,6 +202,9 @@ const flags_ui::FeatureEntry kFeatureEntries[] = {
     {"new-pass-kit-download", flag_descriptions::kNewPassKitDownloadName,
      flag_descriptions::kNewPassKitDownloadDescription, flags_ui::kOsIos,
      FEATURE_VALUE_TYPE(web::features::kNewPassKitDownload)},
+    {"new-file-download", flag_descriptions::kNewFileDownloadName,
+     flag_descriptions::kNewFileDownloadDescription, flags_ui::kOsIos,
+     FEATURE_VALUE_TYPE(web::features::kNewFileDownload)},
     {"ios-share-canonical-url", flag_descriptions::kShareCanonicalURLName,
      flag_descriptions::kShareCanonicalURLDescription, flags_ui::kOsIos,
      FEATURE_VALUE_TYPE(activity_services::kShareCanonicalURL)},